Email bounce codes, explained
Every bounce says why.
In three numbers.
An email bounce code is the receiving server’s reason for refusing your message. A code starting with 5, such as 550 5.1.1, is a hard bounce: permanent. A code starting with 4, such as 421 4.7.28, is a soft bounce: try again later.
The enhanced code after it, class.subject.detail, says whose problem it is. Many 5xx bounces are not about the address at all but about your sending, so removing the address is not always the fix. Paste yours into the decoder below.
By the Builderify team
Hard vs soft bounces:
what 5xx and 4xx mean.
The first digit decides it. SMTP, the protocol mail servers speak (RFC 5321), calls a 4 a transient failure: the request may succeed if it is tried again. A 5 is permanent: sending the same thing again will get the same answer.
Email marketers call those soft and hard bounces. A soft bounce is usually never seen: your sending server keeps the message and tries again for a few days. You only get a bounce message when it finally gives up, or when the answer was a 5 in the first place.
Soft bounce
- Typical causes
- Mailbox full, server busy, too much mail too fast, a reputation or authentication warning
- What happens
- Your server holds the message and retries on its own
- What you do
- Keep the address. Slow down if it is a rate limit
Hard bounce
- Typical causes
- Address does not exist, domain has no mail server, or your mail was refused by policy
- What happens
- Your server stops and sends you a bounce message
- What you do
- Remove the address only if the problem is the address
A hard bounce is not always a bad address. Codes in the 5.7.x family mean the receiving server refused you: authentication, reputation or policy. Deleting those recipients fixes nothing, and the next send bounces the same way.
How to read a bounce message
Find the line that starts with three digits. It is usually after “The response was”, “Diagnostic-Code” or “Remote server returned”. It has up to three parts.
-
01 · Basic reply code
Three digits from RFC 5321. The first decides hard or soft.
550and554are the usual permanent refusals;421and450the usual temporary ones. -
02 · Enhanced status code
class.subject.detail, from RFC 3463. Class repeats hard or soft, the subject says which part of the system failed, and the detail narrows it down. -
03 · Provider text
The receiving provider’s own explanation, often with a help link. Providers reuse the same code for different reasons, so this part is what you search for.
The middle number: RFC 3463’s eight subjects
- X.0
- Other or undefined Something went wrong that the code does not classify.
- X.1
- Addressing A problem with the recipient's address: the mailbox or domain does not exist, or the address is malformed.
- X.2
- Mailbox The address exists, but the mailbox cannot take mail right now: full, disabled or over a limit.
- X.3
- Mail system The receiving mail system has a problem of its own, such as a full disk or a message too big for it.
- X.4
- Network and routing Something between the servers failed: DNS, routing, a connection, or a message that waited too long.
- X.5
- Mail delivery protocol The two servers disagreed about the SMTP conversation itself.
- X.6
- Message content or media The receiving server could not handle the message as written: its format, encoding or attachments.
- X.7
- Security or policy A security or policy refusal: authentication, reputation, a block list or the recipient's own rules.
Bounce message
decoder.
Paste the whole bounce, headers and all. The decoder finds the reply code and the enhanced code, works out which provider sent it, and looks it up in the table below. Codes it does not know get the general meaning of their class and subject from RFC 3463.
- Hard or soft, in plain English
- Whose problem it is: the address, the mailbox or your sending
- The fix, and whether to retry or remove the address
- Runs in your browser; nothing you paste is sent anywhere
Unauthenticated sender
Gmail could not confirm you are allowed to send for your domain. Gmail uses the same code for an SPF hard fail and for mail refused under the sending domain's own DMARC policy.
The problem is on your side. Fix it, then send again.
The address is fine. Fix SPF, DKIM and DMARC for the domain you send from, check a test message passes, then send again.
How to set up SPF, DKIM and DMARC →
- Class
- 5 = permanent failure
- Subject
- 7 = Security or policy
- Detail
- 26
Matched a verified entry in the table below.
SMTP error codes, explained in one table
Provider wording as published · checked 3 Oct 2026
| Code | Provider | What it says | What it means | Type | What to do |
|---|---|---|---|---|---|
| 550 5.7.26 | Gmail | This email has been blocked because the sender is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM. |
Unauthenticated sender. Gmail could not confirm you are allowed to send for your domain. Gmail uses the same code for an SPF hard fail and for mail refused under the sending domain's own DMARC policy. | Hard | Keep the address, fix your sending. The address is fine. Fix SPF, DKIM and DMARC for the domain you send from, check a test message passes, then send again. Setup guide |
| 421 4.7.26 | Gmail | This email has been rate limited because it is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM. |
Rate limited: unauthenticated. The temporary form of 5.7.26. Gmail is holding back mail it cannot authenticate; left unfixed, it turns into rejections. | Soft | Keep the address, fix your sending. The address is fine. Fix SPF, DKIM and DMARC for the domain you send from, check a test message passes, then send again. Setup guide |
| 421 4.7.27 | Gmail | Your email has been rate limited because SPF authentication didn't pass for this message. Gmail requires all bulk email senders to authenticate their email with SPF. |
Rate limited: SPF did not pass. SPF failed for the domain in your return path, so Gmail is slowing your mail down. | Soft | Keep the address, fix your sending. The address is fine. Fix SPF, DKIM and DMARC for the domain you send from, check a test message passes, then send again. Setup guide |
| 550 5.7.27 | Gmail | This message was blocked because it didn't pass SPF authentication. |
Blocked: SPF did not pass. SPF failed and Gmail refused the message. (In RFC 7505, X.7.27 means something else, a null MX; Gmail uses it for SPF.) | Hard | Keep the address, fix your sending. The address is fine. Fix SPF, DKIM and DMARC for the domain you send from, check a test message passes, then send again. Setup guide |
| 421 4.7.30 | Gmail | Your email has been rate limited because DKIM authentication didn't pass for this message. |
Rate limited: DKIM did not pass. Your message was not signed with DKIM, or the signature did not verify. | Soft | Keep the address, fix your sending. The address is fine. Fix SPF, DKIM and DMARC for the domain you send from, check a test message passes, then send again. Setup guide |
| 550 5.7.30 | Gmail | This message was blocked because it didn't pass DKIM authentication. |
Blocked: DKIM did not pass. Your message was not signed with DKIM, or the signature did not verify, and Gmail refused it. | Hard | Keep the address, fix your sending. The address is fine. Fix SPF, DKIM and DMARC for the domain you send from, check a test message passes, then send again. Setup guide |
| 421 4.7.32 | Gmail | Your email has been rate limited because the From: header (RFC5322) in this message isn't aligned with either the authenticated SPF or DKIM organizational domain. |
Rate limited: From not aligned. SPF or DKIM may pass, but for a different domain from the one in your From address, so DMARC alignment fails. | Soft | Keep the address, fix your sending. The address is fine. Fix SPF, DKIM and DMARC for the domain you send from, check a test message passes, then send again. Setup guide |
| 5.7.32 | Gmail | Your email was blocked because the From: header (RFC5322) in this message isn't aligned with either the authenticated SPF or DKIM organizational domain. |
Blocked: From not aligned. SPF or DKIM may pass, but not for your From domain, and Gmail refused the message. (Google's table lists this with a 421 reply code.) | Hard | Keep the address, fix your sending. The address is fine. Fix SPF, DKIM and DMARC for the domain you send from, check a test message passes, then send again. Setup guide |
| 421 4.7.40 | Gmail | Your email has been rate limited because the sending domain doesn't have a DMARC record, or the DMARC record doesn't specify a DMARC policy. |
Rate limited: no DMARC record. Your domain publishes no DMARC record, or one without a p= policy. | Soft | Keep the address, fix your sending. The address is fine. Fix SPF, DKIM and DMARC for the domain you send from, check a test message passes, then send again. Setup guide |
| 550 5.7.40 | Gmail | Your message was blocked because the sending domain doesn't have a DMARC record or the DMARC record doesn't specify a DMARC policy. |
Blocked: no DMARC record. Your domain publishes no DMARC record, or one without a p= policy, and Gmail refused the message. | Hard | Keep the address, fix your sending. The address is fine. Fix SPF, DKIM and DMARC for the domain you send from, check a test message passes, then send again. Setup guide |
| 550 5.7.25 | Gmail | This message was blocked because the sending IP address doesn't have a PTR record, or the forwarding DNS entry doesn't reference the sending IP address. |
No PTR record for the sending IP. The server that sent your mail has no reverse DNS name, or the name does not point back to the same IP. | Hard | Keep the address, fix your sending. The address is fine. The PTR record belongs to whoever owns the sending IP, your host or email provider, so ask them to set it. |
| 451 4.7.23 | Gmail | The sending IP address for this message doesn't have a PTR record, or the PTR record's forward DNS entry doesn't match the sending IP address. To protect users from spam, your email has been temporarily rate limited. |
Rate limited: no PTR record. The temporary form of 5.7.25: reverse DNS for your sending IP is missing or does not match. | Soft | Keep the address, fix your sending. The address is fine. Ask whoever owns the sending IP to set a PTR record that resolves back to it. |
| 421 4.7.28 | Gmail | Gmail has detected an unusual rate of email. To protect our users from spam, email has been temporarily rate limited. |
Rate limited: unusual rate of email. Gmail is slowing mail from your IP, domain or links because of a sudden jump in volume or mail its users treat as spam. Your server retries. | Soft | Keep the address, let it retry. The address is fine. Send in smaller batches, stop mailing people who never open, and check your spam rate in Postmaster Tools. |
| 550 5.7.28 | Gmail | There is an unusual rate of unsolicited email originating from your IP address. To protect our users from spam, email sent from your IP address has been blocked. |
Blocked: unusual rate of unsolicited email. The permanent form of 4.7.28: Gmail has stopped accepting this mail from your IP. | Hard | Keep the address, fix your sending. The address is fine. Pause, clean the list down to people who asked for your mail, and send less at once before trying again. |
| 421 4.7.0 | Gmail | This message is suspicious due to the very low reputation of the sending IP address. |
Temporary refusal. Gmail uses 4.7.0 for several temporary refusals: low IP or domain reputation, a missing PTR record, or a domain that requires TLS. The words after the code say which. | Soft | Keep the address, let it retry. The address is fine. Let your server retry, and act on the reason in the text. |
| 550 5.7.1 | Gmail | This message is likely unsolicited email. To reduce the amount of spam sent to Gmail, this message has been blocked. |
Blocked as likely spam, or by policy. Gmail refused this message on content or reputation grounds. The same code also covers a recipient domain whose policy forbids your mail. | Hard | Keep the address, fix your sending. The address is usually fine. Read the text after the code, and check authentication, links and complaint rate before sending again. |
| 550 5.7.29 | Gmail | This message was blocked because it wasn't sent over a TLS connection. |
Blocked: no TLS. Your server delivered the message without encryption, and Gmail now refuses that for this mail. | Hard | Keep the address, fix your sending. The address is fine. Turn on TLS for outgoing mail on your sending server, or send through a provider that uses it. |
| 552 5.3.4 | Gmail | Your message exceeded Google's message size limits. |
Message too big. The message, attachments included, is larger than Gmail accepts. | Hard | Keep the address, fix your sending. The address is fine. Link to large files instead of attaching them, then send again. |
| 550 5.1.1 | Gmail | The email account that you tried to reach does not exist. Please double-check the recipient's email address for typos or unnecessary spaces. |
The address does not exist. There is no Gmail account at that address: it was mistyped, made up or deleted. | Hard | Remove the address. Correct it if there is an obvious typo. Otherwise remove it: retrying gets the same answer. |
| 550 5.2.1 | Gmail | The email account that you tried to reach is inactive. |
Inactive account. The Gmail account exists but is no longer in use. | Hard | Remove the address. Remove it from your list. |
| 552 5.2.2 | Gmail | The recipient's inbox is out of storage space and inactive. |
Inbox full and inactive. The mailbox is full and nobody is using it. | Hard | Remove the address. Remove it from your list. |
| 452 4.2.2 | Gmail | The recipient's inbox is out of storage space. |
Inbox out of storage. The mailbox is full right now. The address is real. | Soft | Keep it for now, remove if it repeats. Keep it for now. If it is still full on the next few sends, remove it. |
| 450 4.2.1 | Gmail | The user you are trying to contact is receiving email too quickly. Please resend your message at a later time. |
Recipient receiving mail too fast. This one recipient is getting more mail than Gmail will accept right now. | Soft | Keep the address, let it retry. Keep the address. Your server retries later on its own. |
| 550 5.7.515 | Outlook / Microsoft 365 | Access denied, sending domain [SendingDomain] does not meet the required authentication level. |
Domain below Outlook's required authentication level. Outlook.com refused mail from a high-volume sender whose domain does not pass SPF, DKIM and an aligned DMARC record. | Hard | Keep the address, fix your sending. The address is fine. Fix SPF, DKIM and DMARC for the domain you send from, check a test message passes, then send again. Setup guide |
| 550 5.7.509 | Outlook / Microsoft 365 | Access denied, sending domain [$SenderDomain] does not pass DMARC verification and has a DMARC policy of reject. |
Failed DMARC, policy is reject. Your From domain says reject anything that fails DMARC, and this message failed it. | Hard | Keep the address, fix your sending. The address is fine. Fix SPF, DKIM and DMARC for the domain you send from, check a test message passes, then send again. Setup guide |
| 550 5.7.23 | Outlook / Microsoft 365 | The message was rejected because of Sender Policy Framework violation |
SPF violation. The server that sent your mail is not listed in your domain's SPF record. | Hard | Keep the address, fix your sending. The address is fine. Fix SPF, DKIM and DMARC for the domain you send from, check a test message passes, then send again. Setup guide |
| 550 5.7.1 | Outlook / Microsoft 365 | Delivery not authorized |
Delivery not authorized. In Microsoft 365, the sender is not allowed to send to this recipient: often a group or mailbox that only accepts mail from certain people. | Hard | Read the provider text, then decide. Read the text after the code. If the recipient only accepts internal mail, no list change will help; remove them if they will never accept yours. |
| 550 5.7.606–649 | Outlook / Microsoft 365 | Access denied, banned sending IP [IP1.IP2.IP3.IP4] |
Banned sending IP. Microsoft has blocked the IP address your mail came from. | Hard | Keep the address, fix your sending. The address is fine. The block is on the sending IP: if you run the server, follow Microsoft's delisting steps; if you use a provider, tell them. |
| 451 4.7.500–699 | Outlook / Microsoft 365 | Access denied, please try again later |
Sending temporarily restricted. Microsoft has temporarily restricted mail from your server, usually over suspicious volume. | Soft | Keep the address, let it retry. The address is fine. Let your server retry, and send at a steadier pace. |
| 5.1.1–20 | Outlook / Microsoft 365 | …generally indicate that the recipient can't be found or that message routing information is invalid. |
Recipient not found. Microsoft groups 5.1.1 to 5.1.20 together: the address does not exist in the receiving organisation, or its routing is broken. | Hard | Remove the address. Check for a typo. Otherwise remove the address. |
| 550 5.4.1 | Outlook / Microsoft 365 | Recipient address rejected: Access denied |
Recipient address rejected. Microsoft gives the cause as an address that does not exist in the receiving organisation. | Hard | Remove the address. Check for a typo. Otherwise remove the address. |
| 5.2.2 | Outlook / Microsoft 365 | Submission quota exceeded |
Submission quota exceeded. Unlike Gmail's 5.2.2, this one is about the sender: the sending Microsoft 365 account went over its sending limits. | Hard | Keep the address, fix your sending. The address is fine. Wait for the limit to reset and send less at once. |
| 421 RP-001 | Outlook / Microsoft 365 | The mail server IP connecting to Outlook.com server has exceeded the rate limit allowed. Reason for rate limitation is related to IP/domain reputation. |
Outlook.com rate limit (RP-001). Outlook.com is slowing mail from your IP because of its reputation. | Soft | Keep the address, let it retry. The address is fine. Let your server retry, send less at once and to more engaged people. |
| 550 SC-001 | Outlook / Microsoft 365 | Mail rejected by Outlook.com for policy reasons. Reasons for rejection may be related to content with spam-like characteristics or IP/domain reputation. |
Outlook.com policy block (SC-001). Outlook.com refused the message over its content or your reputation. | Hard | Keep the address, fix your sending. The address is fine. Review content, links and list quality before sending again. |
| 550 SC-004 | Outlook / Microsoft 365 | … A block has been placed against your IP address because we have received complaints concerning mail coming from that IP address. |
Outlook.com complaint block (SC-004). Enough Outlook.com users complained about mail from your IP that it is now blocked. | Hard | Keep the address, fix your sending. The address is fine. Find out why people complain, usually mail they did not ask for, and stop sending it. |
| TS errors | Yahoo / AOL | This is a temporary error and your mail server may automatically re-try sending the email at a later time. |
Message temporarily deferred (TS errors). Yahoo is holding back your mail for now. It lists complaints, objectionable content, poor IP reputation and unusual traffic patterns as causes. | Soft | Keep the address, let it retry. The address is fine. Let your server retry, slow down, and mail engaged subscribers first. |
| PH errors | Yahoo / AOL | These error messages indicates that your email wasn't accepted because there is something in the content that Yahoo won't accept for policy reasons. |
Content-based block (PH errors). Something in the message, Yahoo names viruses, phishing, malicious software and links to them, made Yahoo refuse it. | Hard | Keep the address, fix your sending. The address is fine. Check every link and attachment in the message, then send again. |
| 554 dd | Yahoo / AOL | The Yahoo account that you're trying to send to does not exist. |
Recipient does not exist. There is no Yahoo account at that address. Bounces show it as "554 delivery error: dd This user doesn't have a yahoo.com account". | Hard | Remove the address. Remove the address and do not retry, as Yahoo itself advises. |
| 5.1.1 | RFC 3463 | Bad destination mailbox address |
Bad destination mailbox address. RFC 3463: the part before the @ does not exist on the receiving system. | Hard | Remove the address. Check for a typo. Otherwise remove the address. |
| 5.1.10 | RFC 7505 | Recipient address has null MX |
Recipient address has null MX. RFC 7505: the recipient's domain publishes a "null MX", saying it accepts no mail at all. | Hard | Remove the address. Remove the address. |
| 5.2.2 | RFC 3463 | Mailbox full |
Mailbox full. RFC 3463: the mailbox is over its quota. The RFC says this should be treated as a temporary failure, even when it arrives as a 5. | Hard | Keep it for now, remove if it repeats. Keep it for now. If it is still full on the next few sends, remove it. |
| 5.4.7 | RFC 3463 | Delivery time expired |
Delivery time expired. RFC 3463: your server kept retrying a temporary failure until it gave up. The original reason is often further up the bounce. | Hard | Keep it for now, remove if it repeats. Look for the earlier 4xx reason. If the domain never answers, remove the address. |
| 5.7.1 | RFC 3463 | Delivery not authorized, message refused |
Delivery not authorized, message refused. RFC 3463: the receiving server refused on policy grounds. Providers use it for block lists, reputation, content and their own rules. | Hard | Keep the address, fix your sending. The address is usually fine. The text after the code names the policy; start there. |
| 5.7.23 | RFC 7372 | SPF validation failed |
SPF validation failed. RFC 7372: the server that sent your mail is not allowed by your domain's SPF record. | Hard | Keep the address, fix your sending. The address is fine. Fix SPF, DKIM and DMARC for the domain you send from, check a test message passes, then send again. Setup guide |
| 5.7.25 | RFC 7372 | Reverse DNS validation failed |
Reverse DNS validation failed. RFC 7372: the sending IP has no matching PTR record. | Hard | Keep the address, fix your sending. The address is fine. Ask whoever owns the sending IP to fix its PTR record. |
| 5.7.26 | RFC 7372 | Multiple authentication checks failed |
Multiple authentication checks failed. RFC 7372: the message failed more than one authentication check, such as SPF and DKIM. | Hard | Keep the address, fix your sending. The address is fine. Fix SPF, DKIM and DMARC for the domain you send from, check a test message passes, then send again. Setup guide |
| 5.7.27 | RFC 7505 | Sender address has null MX |
Sender address has null MX. RFC 7505: the domain you send from says it accepts no mail, so bounces could never reach you. (Gmail uses 5.7.27 for SPF failures instead.) | Hard | Keep the address, fix your sending. The address is fine. Send from a domain that can receive mail, including its bounces. |
Sources: Gmail SMTP errors and codes, Outlook’s high-volume sender requirements, Exchange Online NDRs, Outlook.com postmaster, Yahoo Sender Hub, and RFCs 3463, 7372 and 7505. Yahoo publishes categories rather than individual codes. Providers change their wording; the code is the stable part.
550 5.7.26, 550 5.7.515
and other authentication bounces.
These bounce every recipient at that provider at once, and none of them is about the recipient. The receiving server could not prove the mail really came from your domain. The fix is DNS, not your list: set up SPF, DKIM and DMARC for the domain you send from.
Gmail: the sender is unauthenticated
“This email has been blocked because the sender is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM.”
Gmail uses the same code when SPF hard-fails and when your domain's own DMARC policy says reject. Its softer siblings, 421 4.7.26, 4.7.27 (SPF), 4.7.30 (DKIM), 4.7.32 (From alignment) and 4.7.40 (no DMARC), rate-limit instead, and each has a 550 version that blocks.
Source →Outlook: the domain does not meet the authentication level
“Access denied, sending domain [SendingDomain] does not meet the required authentication level.”
Outlook.com's rule for domains sending over 5,000 messages a day to its consumer addresses: SPF must pass, DKIM must pass, and DMARC must be at least p=none and aligned with one of them. Exchange Online has its own: 5.7.23 for SPF and 5.7.509 for a DMARC reject.
Source →Gmail: no reverse DNS for your server
“This message was blocked because the sending IP address doesn't have a PTR record, or the forwarding DNS entry doesn't reference the sending IP address.”
Not a record you can add on your own domain. The PTR belongs to whoever owns the sending IP, your host or email provider. Gmail's temporary version is 451 4.7.23.
Source →Why these codes got common
-
Feb 2024Gmail and Yahoo
Gmail requires senders of 5,000 or more messages a day to authenticate their mail, avoid unwanted mail and make unsubscribing easy; Yahoo brings in matching rules. Google says enforcement will be gradual and progressive. Source
-
5 May 2025Outlook.com
Microsoft first said non-compliant mail from domains sending over 5,000 a day would go to Junk. An update on 29 April 2025 changed that to rejection, 550 5.7.515, from 5 May. Its postmaster page still describes Junk first. Source
-
Nov 2025Gmail steps up
In Google's words, Gmail is “ramping up its enforcement on non-compliant traffic”: mail that misses the requirements sees disruptions, including temporary and permanent rejections. Source
Got one of these? Do not touch the list. Fix the records, check them with a test send, and resend to the same people. The step-by-step is in our SPF, DKIM and DMARC setup guide.
421 4.7.28, 421 4.7.0, TSS04:
deferred, not refused.
A 421 is the receiving server saying “not now”. Your mail is not lost: your sending server keeps it and tries again. What it tells you is that the provider has started to distrust the volume or the reputation of your mail, and it will turn into hard refusals if nothing changes.
421 4.7.28 Gmail: an unusual rate of email
In Gmail’s words: “Gmail has detected an unusual rate of email. To protect our users from spam, email has been temporarily rate limited.” Variants of the message name what it is counting: your IP, your netblock, your DKIM or SPF domain, or a domain in your links. Send in smaller batches, stop mailing people who never open, and check your spam rate in Postmaster Tools. Ignored, it becomes 550 5.7.28, a block.
421 4.7.0 Gmail: try again later
One code, several reasons. Gmail’s table lists it for a sending IP or domain with very low reputation, an IP without a PTR record, and a recipient domain that requires TLS or only accepts listed IPs. Your server retries; the words after the code tell you which of those to fix.
421 RP-001 Outlook.com: rate limited
Outlook.com uses its own short codes. RP-001 means your IP “has exceeded the rate limit allowed”, and Microsoft gives the reason as IP or domain reputation. In Exchange Online, the equivalent temporary block is 451 4.7.500 to 4.7.699, “Access denied, please try again later”.
421 · TS errors Yahoo: message temporarily deferred
Yahoo tags its deferrals with codes starting TS, such as [TSS04]. Its own description: “This is a temporary error and your mail server may automatically re-try sending the email at a later time.” It lists complaints, objectionable content, poor IP reputation and unusual traffic patterns as causes.
550 5.7.1 Refused as spam, or by policy
The permanent end of the same road. RFC 3463 calls 5.7.1 “delivery not authorized, message refused”, and providers hang very different reasons on it: Gmail for “likely unsolicited email” or a recipient domain whose policy forbids your mail, Microsoft 365 for a recipient that only accepts mail from certain senders. The text after the code is what matters.
- 00:00 421 4.7.28 Kept in the queue Deferred
- 00:30 421 4.7.28 Retry, at least 30 minutes later Deferred
- 02:00 421 4.7.28 Retry Deferred
- 06:00 250 2.0.0 Accepted: no bounce ever sent Delivered
4.4.7 or 5.4.7: “delivery time expired” in RFC 3463. RFC 5321 suggests retrying at intervals of at least 30 minutes, for at least 4–5 days.
Illustrative times. Each sending server sets its own schedule within those limits.
550 5.1.1 and 552 5.2.2:
when to stop mailing an address.
Only two families of codes are really about the address: subject 1, addressing, and subject 2, mailbox. Those are the ones that should change your list. Mailing addresses that keep bouncing tells providers you do not look after it, and that costs you delivery to everyone else.
550 5.1.1 User unknown
The address does not exist. Gmail says “The email account that you tried to reach does not exist”. Microsoft groups 5.1.1 to 5.1.20 together as a recipient that “can’t be found”, and Yahoo’s version reads “dd This user doesn’t have a yahoo.com account”. Look for an obvious typo, gmial.com and the like, and correct it if you can. Otherwise remove it today: Yahoo’s own advice is to remove the address and not retry.
552 5.2.2 Mailbox full
The mailbox is over its storage quota. RFC 3463 says a full mailbox should be treated as temporary, and Gmail sends exactly that as a soft 452 4.2.2: “The recipient’s inbox is out of storage space.” Its hard 552 5.2.2 adds two words, “and inactive”: nobody reads it, so remove it. One trap: in Microsoft 365, 5.2.2 means “Submission quota exceeded”, a limit on the sender, not a full mailbox.
When to remove an address
5.1.x: user unknown, bad address, no such domain
The address will never work. One hard bounce is enough.
4.2.x, and 5.2.2 from most servers: mailbox full or over a limit
One failure can be temporary. The same address failing on send after send is not. Inactive accounts, such as Gmail's 5.2.1, go at once.
5.7.x: authentication, reputation, block lists, policy
Every recipient at that provider would bounce the same way, so the list is not the problem. Never resend the same message unchanged, though: Yahoo says plainly not to retry a 5xx.
4.7.x and other 4xx: rate limits and deferrals
Your server retries. Send less at once and to more engaged people.
Unsubscribes and spam complaints
Not bounces, but the same rule: stop mailing them. Gmail and Yahoo expect one-click unsubscribes honoured within two days.
How Builderify handles
bounces for you.
Broadcast sends campaigns, and Sequences automated follow-ups, from your own verified domain. The bounces come back to that domain too, and you never have to read them.
-
01
Sent from your domain
Every message goes out with your domain's bounce address as its return path, and carries a one-click unsubscribe header.
-
02
Refused somewhere
The receiving server answers with a code, and the bounce report is sent back to that return path.
-
03
Back to Builderify
The MX record you add when you connect the domain routes it to Inbox, which files bounces, unsubscribes and abuse reports apart from real replies.
-
04
Matched to the contact
The contact is marked bounced. Later broadcasts skip it, and it leaves any sequence it is in.
-
No suppression list to keep
Bounced and unsubscribed contacts stay in your lead lists, marked as such, and are left out of every later send. Builderify does not wait for a second bounce: one bounce report is enough to mark the contact.
-
Bounces you can see per campaign
Each campaign shows recipients, sent, opened, bounced and failed counts, so a send that suddenly bounces more than usual stands out.
-
One-click unsubscribe
Every broadcast and sequence email carries the List-Unsubscribe and List-Unsubscribe-Post headers from RFC 8058, so Gmail and Yahoo can show their own unsubscribe button. Opt-outs are recorded against the contact.
-
Complaints filed apart
Spam complaints sent to your domain's abuse address are recorded separately rather than mixed into your Inbox with real replies.
-
Authentication checked before you send
Sending unlocks only when the domain's ownership, MX, SPF and DKIM records check out; DMARC is recommended. The records themselves are in the SPF, DKIM and DMARC guide.
No subscription. If you outgrow the free credits, one-time packs never expire. The free credits are shared with any other module you use; the full rate card is on the pricing page.
Start freeHandling bounces well keeps your list clean; it does not guarantee the inbox. Mail people asked for, sent from an authenticated domain, still does most of the work. Broadcast is one of 19 modules on the same account; see them all.
Email bounce codes, before you ask.
A hard bounce is a permanent failure: the code starts with 5, such as 550 5.1.1, and sending the same message again gets the same answer. A soft bounce is temporary: the code starts with 4, such as 421 4.7.28, and the sending server keeps the message and retries. Not every hard bounce means a bad address: 5.7.x codes are about your authentication or reputation.
Gmail blocked the message because the sender is unauthenticated: SPF and DKIM both failed, or your domain's own DMARC policy said reject. The recipient's address is fine, so do not remove it. Fix SPF, DKIM and DMARC for the domain you send from, then send again.
It is Outlook.com's rejection for domains sending over 5,000 messages a day whose authentication falls short: "Access denied, sending domain does not meet the required authentication level." Since 5 May 2025 Microsoft rejects that mail. SPF and DKIM must pass, and DMARC must be at least p=none and aligned with one of them.
Remove it after one bounce saying the address does not exist, such as 550 5.1.1 or Yahoo's "dd This user doesn't have a yahoo.com account". A full mailbox can be temporary, so keep it unless it keeps failing, but remove inactive accounts at once. Do not remove addresses over 5.7.x or 4.7.x codes: those are about your sending, and every recipient at that provider would bounce the same way.
Yes. Broadcast and Sequences send from your own verified domain, and the MX record you add routes bounce reports back to Builderify. A bounce marks the contact as bounced, so later broadcasts skip it and it leaves any sequence it is in, with no suppression list to keep. Each campaign shows its bounced count.
Sources
- Google Gmail SMTP errors and codes
- Google Email sender guidelines
- Google Email sender guidelines FAQ
- Microsoft Outlook's new requirements for high-volume senders
- Microsoft Outlook.com postmaster troubleshooting
- Microsoft Learn Non-delivery reports in Exchange Online
- Microsoft Learn Fix NDR 550 5.1.1 to 5.1.20 in Exchange Online
- Yahoo Sender Hub: SMTP error codes
- IETF RFC 5321: Simple Mail Transfer Protocol
- IETF RFC 3463: Enhanced Mail System Status Codes
- IETF RFC 7372: Email Authentication Status Codes
- IETF RFC 7505: A "Null MX" No Service Resource Record
- IETF RFC 8058: One-click unsubscribe
Codes and wording checked on each provider’s own pages on 3 October 2026. ← All guides
Stop losing the visitors
you already paid for.
More leads from the traffic you already have, and follow-up that sends itself. Free to start. No card, no monthly fee.