TL;DR: SMTP error codes tell you why an email didn't get through. 4xx codes are
temporary ("try again later"); 5xx codes are permanent ("don't bother retrying").
The most common is 550 — a permanent rejection, often a bad address or a policy/auth block. Below is what each
common code means and what to do. Scan your domain free if you suspect an authentication cause.
How SMTP codes are structured
The first digit is what matters most: 2xx = success, 4xx = temporary failure (a soft bounce; the server should retry), 5xx = permanent failure (a hard bounce). The text after the code gives the detail.
Common 4xx (temporary) codes
| Code | Meaning & action |
|---|---|
421 | Service unavailable / too many connections — your server retries later. |
450 | Mailbox temporarily unavailable (busy or locked) — retry. |
451 | Local error / often greylisting — retry shortly. |
452 | Insufficient storage (mailbox full) or too many recipients — retry / reduce. |
Common 5xx (permanent) codes
| Code | Meaning & action |
|---|---|
550 | Mailbox unavailable / rejected by policy — bad address, or blocked for spam/auth. Remove or fix the cause. |
551 | User not local — recipient moved; update the address. |
552 | Message too large / mailbox over quota — shrink the message. |
553 | Invalid or disallowed address format — check the recipient address. |
554 | Transaction failed — often a spam/blocklist or authentication rejection. |
When a 5xx is really an auth problem
Many 550/554 rejections say "message rejected" or "policy" — and the underlying cause is failing
SPF/DKIM/DMARC or a blacklist.
If you're getting these on mail to valid addresses, check your authentication first.
FAQ
Should I retry a 5xx error?
No — 5xx is permanent. Fix the cause (address, size, auth, reputation) rather than resending.
What's the difference between 4xx and 5xx?
4xx is temporary (retry); 5xx is permanent (don't). It mirrors soft vs hard bounces.
I keep getting 550 to good addresses — why?
Usually authentication or reputation. Check SPF/DKIM/DMARC and whether you're blacklisted.
Getting 550s you can't explain? Scan your domain, then reply to your report — we're developers and we'll check whether authentication is the cause and fix it.