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A free reference by Deliveryman

SMTP Error Codes and Email Bounce Messages, Decoded

Look up any SMTP bounce code and know exactly what to do next. A free reference of 206+ bounce responses returned by 21 email providers and 8 spam filters. It captures the wording Gmail, Outlook, Yahoo, Proofpoint and others use when a message is deferred or rejected, and pairs each one with a plain-English meaning, a severity rating and a fix.

Search a status code, an email provider, or the text of the bounce you got.

Start typing to search 41 codes, providers and spam filters.

Browse by SMTP status code

The numeric reply is the starting point. Open a code to see every provider and spam-filter variation on record.

4.x.x transient failure

A temporary problem. The sending server keeps retrying for a while, and delivery often recovers on its own once the underlying issue clears.

5.x.x permanent failure

A hard failure. The receiving server has refused the message for good and will not accept an unchanged retry without a change on the sending side.

Browse by email provider

Mailbox providers and hosts: the servers that deliver to an inbox.

Browse by spam filter

Gateways and security services that sit in front of an inbox and can reject mail before the provider sees it.

How to read an SMTP bounce message

A bounce, also called a non-delivery report, has three parts worth reading in order.

1. The SMTP reply code

The three-digit number. 4xx is temporary and will be retried. 5xx is permanent. This tells you whether to wait or to act.

2. The enhanced status code

The dotted number such as 5.7.1 or 5.1.1 (RFC 3463). The middle digit points at the subject: .1 addressing, .2 mailbox, .4 network or routing, .7 policy or security.

3. The free-text explanation

The human sentence and any URL. This is where the receiver tells you the real reason, whether that is a missing SPF record, a DMARC failure, a blocklisted IP, or a typo in the address. Every page here pairs the verbatim response with a plain-English summary and links to the receiver’s own documentation.

Common questions

›What is an SMTP error code?

It is the three-digit reply a receiving mail server returns when it accepts, defers, or rejects a message. A code that starts with 4 (such as 421 or 450) is a temporary failure the sender will retry. A code that starts with 5 (such as 550 or 554) is a permanent failure that will not be retried unchanged.

›What is the difference between a 4xx and a 5xx SMTP reply?

A 4xx reply is a temporary failure. The message is queued and retried for a while, and it often delivers on its own. A 5xx reply is a permanent failure. The receiver has refused the message, and the same message will not be accepted on a retry.

›Why does the same SMTP code look different at Gmail, Outlook and Yahoo?

The standard only fixes the number. Each provider and spam filter writes its own message text, attaches its own enhanced status code such as 5.7.1, and links to its own documentation. That is how one 550 can mean an unknown address at one receiver and a reputation block at another.

›How do I fix an SMTP bounce?

Read the enhanced status code and the message text, not only the number. Wording about the recipient (user unknown, mailbox full) means fix or drop the address. Wording about the sender (SPF, DKIM, DMARC, reputation, blocklist) means fix your authentication or sending practices, or clear the listing.