SMTP error codes and bounce messages
When an email bounces, the receiving server returns a code such as 550 5.1.1. Find yours below to see what it means, why it happens, how to fix it and what to do with the address.
The first digit tells you the kind of failure: 2 means success, 4 a temporary problem to retry, 5 a permanent failure. The numbers after the dot (the enhanced status code) say why.
Permanent failure (hard bounce)
The address or domain is wrong or gone. Retrying will not help; remove the address.
- 550 5.1.1 user unknown (mailbox does not exist)The receiving server says there is no mailbox with that name at the domain.
- 550 5.1.2 bad destination system address (domain not found)The domain in the address does not exist or cannot receive mail.
- 550 5.2.1 mailbox disabled or not accepting messagesThe mailbox exists but has been disabled, so it will not accept mail.
- 552 5.3.4 message too bigThe message is larger than the receiving server allows.
- 550 5.4.1 recipient address rejected: access deniedThe recipient domain's server refused the address, typically because it does not exist in their directory.
- 550 5.5.0 requested action not taken: mailbox unavailableA general permanent refusal: the mailbox is not available for delivery.
- 553 5.1.3 invalid recipient address syntaxThe recipient address is not a valid email address format.
- 501 5.1.3 bad recipient address syntaxThe server could not parse the recipient address you gave it.
- 500 5.5.1 syntax error, command unrecognizedThe server did not understand a command your mail software sent.
- 503 5.5.1 bad sequence of commandsYour software sent SMTP commands in the wrong order, for example sending a message before authenticating.
Temporary failure (soft bounce)
A temporary problem such as a full mailbox or rate limit. Retry, and only remove the address if it keeps failing.
- 552 5.2.2 mailbox full (over quota)The recipient's mailbox is over its storage limit and cannot take new messages.
- 421 4.7.0 try again later (rate limited or temporarily deferred)The receiving server is asking you to slow down or try again later.
- 421 4.7.28 Gmail: unusual rate of unsolicited mailGmail is temporarily rate limiting your mail because it looks suspicious or unauthenticated.
- 450 4.2.1 mailbox temporarily unavailableThe mailbox cannot take mail right now, often because it is receiving too much.
- 451 4.7.1 greylisted or temporarily deferredThe server is deliberately delaying your first message from a new sender.
Rejected by policy or reputation
The server refused your message because of your reputation, authentication or settings. The recipient address may be fine.
- 550 5.7.1 message refused (policy or spam rejection)The server refused your message because of a policy, your reputation or its content.
- 554 5.7.1 service unavailable: client host blockedThe receiving server has blocked your sending IP address or host.
- 550 5.7.26 Gmail: unauthenticated email is not acceptedGmail rejected the message because it failed or lacks SPF, DKIM or DMARC authentication.
- 550 5.7.25 Gmail: sending IP needs reverse DNS (PTR record)Gmail requires the sending IP address to have a matching reverse DNS (PTR) record.
- 530 5.7.0 authentication requiredThe server will not accept mail until you log in.
- 535 5.7.8 authentication credentials invalidThe username or password you gave the SMTP server was rejected.
- 554 5.2.0 transaction failed (generic rejection)A catch-all permanent failure: the server refused the message without giving a specific reason in the code.
Success
The server accepted the message.
Bounces hurt your sender reputation. The best fix is to check addresses before you send or clean your whole list.