Email Bounce Code Decoder

Explain 4xx/5xx SMTP bounce codes and how to fix them.

When mail bounces, the sender gets a failure notice containing an SMTP code like 550 5.1.1, 421 or 5.7.1. Paste the full bounce message and this tool finds the codes and explains whether the issue is temporary or permanent, plus the cause and how to fix it.

It helps you tell whether undelivered mail is a recipient-address problem or a sender reputation/auth problem.

Bounce message / codePaste the full bounce message — codes are extracted automatically.

5.1.1PermanentBad destination mailbox

Recipient address doesn't exist. Check for typos or closed accounts.

550PermanentMailbox unavailable / rejected (permanent)

Address doesn't exist or rejected by policy. Often a typo or closed account.

4xx (temporary) vs 5xx (permanent)

The first digit tells you most of the story. 4xx is retried automatically and often delivers later (e.g. greylisting). 5xx will keep failing until you fix the address or policy issue. The enhanced status code (x.x.x) that follows pinpoints the cause, so read it too.

Common bounce codes

550 5.1.1: the recipient address doesn't exist (typo or closed mailbox). Fix the address.
550 5.7.1 / 5.7.26: policy rejection. A failing SPF/DKIM/DMARC check is the most common cause.
552 5.2.2: the mailbox is over quota (may be temporary).
421 4.7.0: temporary rate limiting or greylisting; it will be retried shortly.

Frequent 5.7.x rejections?

Check your sending domain's SPF/DKIM/DMARC, sending IP reputation and reverse DNS (PTR). Most are sender-trust issues. Verify each record with SPF Record Lookup, DKIM Record Lookup and DMARC Record Lookup; assess all three at once with Email Deliverability Check, and check whether your sending IP is blacklisted with Email Blacklist (DNSBL) Check.

Bounce code → cause → action reference

Here are the SMTP bounce codes and enhanced status codes you hit most often, paired with their cause and the fix. Always read the leading digit first: a 4 means temporary (retried automatically) and a 5 means permanent (you have to resolve it).

Code	Type	Cause	Action
`550 5.1.1`	Permanent	No such user (typo or closed mailbox)	Check the address and resend to the correct one
`550 5.7.1`	Permanent	Blocked by policy or failed auth	Check SPF/DKIM/DMARC and sending IP reputation
`552 5.2.2`	Permanent (sometimes temporary)	Mailbox full / over quota	Recipient must clear the mailbox (sender can't fix)
`421 4.7.0`	Temporary	Temporary rate limit / greylisting	Retry with backoff; it is retried automatically
`451 4.3.0`	Temporary	Temporary server error	Retry shortly; if it persists, check the receiving host
`535 5.7.8`	Permanent	SMTP authentication failed	Re-check sending credentials (or app password)
`554 5.7.1`	Permanent	Rejected as spam	Review content/links and improve reputation & auth

Worked example: reading one code line

Suppose your bounce message contains this line.

Input: 550 5.7.1 ... message rejected due to SPF
Leading 5 → permanent rejection. Resending gives the same result, so the cause must be fixed.
Enhanced code 5.7.1 → policy rejection. The SPF wording points to an authentication failure.
Cause: the sending domain's SPF record does not authorize the sending IP.
Action: add the real sending IP/service to the SPF record, confirm it passes with SPF Record Lookup, then resend.

Frequently asked questions

Is 550 different from 550 5.1.1?

550 is a generic permanent rejection; 5.1.1 specifically means 'no such recipient.' When an enhanced code (x.x.x) is present, prefer it.

Will resending work?

For 4xx, automatic retries usually deliver. For 5xx, resending gives the same result until you fix the cause.

My mail keeps going to spam or getting rejected.

Check the sending domain's SPF/DKIM/DMARC and IP reputation. Failing authentication leads to 5.7.x rejections or spam folder placement.

How do I fix a 5.7.1 policy rejection?

It usually points to failing sender authentication (SPF/DKIM/DMARC) or a reputation problem. First confirm your domain's authentication records are correct and that your sending IP is not on a blacklist.

Is the bounce message I paste sent anywhere?

No. The bounce text you paste is parsed entirely in your browser to extract the codes; it is never transmitted to or stored on any server.