SMTP error codes: what they mean and how to fix them

How SMTP error codes are built

A bounce usually carries two codes and a sentence, like 550 5.1.1 The email account that you tried to reach does not exist.

The basic reply code (550) comes from SMTP itself, RFC 5321. The first digit says how it went: 2 success, 3 send more, 4 temporary failure, 5 permanent failure. The second digit is the category (5 is about the mail system or mailbox) and the third narrows it down.
The enhanced status code (5.1.1) comes from RFC 3463 and reads as class.subject.detail. The class repeats the first digit (4 or 5). The subject says what kind of problem it is: 1 addressing, 2 mailbox, 3 mail system, 4 network and routing, 5 protocol, 6 content, 7 security and policy. The detail is the specific reason, registered with IANA.
The text is written by the receiving server and is often the most useful part. Gmail, Microsoft and Yahoo publish what theirs mean.

4xx vs 5xx, soft vs hard bounces

	4xx (temporary)	5xx (permanent)
Meaning	Not now; try again later	No; sending the same thing again won't work
Examples	`421 4.7.0` try again later, `450` mailbox busy, `452 4.2.2` mailbox full	`550 5.1.1` user unknown, `550 5.7.1` refused, `554 5.7.1` transaction failed
Often called	Soft bounce or deferral	Hard bounce
What the sending server does	Queues and retries for hours or days	Gives up and sends a bounce

Common SMTP error codes

Code	Meaning	About	What to do
550 5.1.1	Bad destination mailbox (user unknown)	Address	Suppress
550 5.1.10	Recipient not found (Microsoft 365)	Address	Suppress
550 5.2.1	Mailbox disabled or inactive	Address	Suppress
552 5.2.2	Mailbox full	Address	Retry later; suppress if it persists
550 5.7.1	Delivery not authorized, message refused	Sender	Fix the cause in the text
554 5.7.1	Transaction failed: relay denied, blocklist, policy	Sender	Fix the cause in the text
550 5.7.26	Multiple authentication checks failed (Gmail: unauthenticated or DMARC)	Sender	Fix SPF, DKIM, DMARC
550 5.7.25	Reverse DNS (PTR) check failed	Sender	Add a PTR record for the sending IP
550 5.7.509	DMARC failed with p=reject (Microsoft)	Sender	Fix DMARC alignment
550 5.7.515	Authentication level not met (Outlook.com)	Sender	Pass SPF, DKIM and DMARC
421 4.7.0	Try again later; often reputation or missing PTR (Gmail)	Sender	Slow down; the server retries
421 4.7.28	Unusual rate of mail (Gmail)	Sender	Reduce volume spikes; the server retries
535 5.7.8	Authentication credentials invalid	Your SMTP login	Fix the username or password
550 permanent failure	Generic permanent refusal	Depends	Read the enhanced code and text
SPF PermError	SPF record can't be evaluated	Sender	Fix the SPF record

Meanings from RFC 3463, RFC 7372, Google, Microsoft and Yahoo. Linked codes have their own page.

How Groundmail handles bounces

Temporary (4xx) replies are retried by the mail server. You get an email.delivery_delayed webhook while it does.
Hard bounces (bounce_type: "hard"): the address can't receive mail. It's suppressed automatically, and later sends to it return 422 instead of bouncing again.
Block bounces (bounce_type: "block"): the receiver refused the sender for reputation or policy. The address is not suppressed and the bounce doesn't count toward your bounce rate.
Every bounce keeps the receiver's full reply in bounce_reason, on the email and on the email.bounced webhook.

Before a send goes out, Groundmail validation also holds back addresses that can't receive mail, such as domains with no mail server, so many hard bounces never happen. Details are in the emails and suppressions docs.

Tools for diagnosing bounces

SPF checker, DKIM checker and DMARC checker for authentication failures.
Blocklist checker for refusals that name a list or reputation.
Email header analyzer to read Authentication-Results on a delivered copy.
MX lookup when every address at one domain bounces.

Frequently asked questions

What's the difference between a 4xx and a 5xx SMTP error?

A 4xx reply is temporary: the receiving server is asking you to try again later, and the sending server retries automatically. A 5xx reply is permanent: the same message won't be accepted if you resend it.

What is an enhanced status code?

The X.Y.Z code after the 3-digit reply, defined in RFC 3463. X is the class (4 temporary, 5 permanent), Y the subject (1 addressing, 2 mailbox, 7 security or policy, and so on) and Z the specific detail.

What's the difference between a soft bounce and a hard bounce?

A soft bounce is a temporary failure, usually a 4xx reply, which the sending server retries. A hard bounce is a permanent failure, a 5xx reply. Among hard bounces, a 5.1.x code means the address is bad and should be suppressed, while a 5.7.x code usually means the sender was refused and the address is fine.

Should I remove an address after a bounce?

Only when the bounce is about the address: unknown user, mailbox disabled, domain with no mail server. Refusals for policy, reputation or authentication are about the sender; fix the cause and keep the address.

Why does my bounce say 550 permanent failure with no detail?

Some sending systems summarise every 5xx as a permanent failure. The full bounce or your provider's logs should include the receiving server's own code and text, which is what tells you the cause.

Sources

Code meanings come from the SMTP standards; receiver-specific wording from each mailbox provider's published documentation.