TL;DR
- The first digit is what matters most: 2 = accepted, 4 = temporary, try later (soft bounce), 5 = permanent (hard bounce).
- The enhanced code narrows it down: x.1.x = address problem, x.2.x = mailbox problem, x.7.x = security or policy, usually about the sender.
550 5.1.1means the mailbox doesn't exist: remove it.550 5.7.1usually means you're blocked: fix your sending, not your list.
How to read a bounce code
A typical rejection looks like this:
550 5.1.1 <[email protected]>: Recipient address rejected: User unknown550 is the basic SMTP reply code from RFC 5321.
5.1.1 is the enhanced status code from RFC 3463, in the form
class.subject.detail. The text after it is free-form and written by the receiving server, and it's often the most useful part.
(Both RFCs checked 2026-10-17.)
Servers don't always pick the most precise code. A block can arrive as a plain 550, and a missing mailbox as
554. When the number and the text disagree, trust the text.
Basic SMTP reply codes
| Code | RFC 5321 meaning | In plain English | What to do |
|---|---|---|---|
250 | Requested mail action okay, completed | Accepted. | Nothing. (On a catch-all domain, "accepted" proves little.) |
251 | User not local; will forward | Accepted and passed on elsewhere. | Nothing. |
252 | Cannot VRFY user, but will accept message and attempt delivery | The server won't confirm the user but will take the mail. | Treat as unconfirmed. |
421 | Service not available, closing transmission channel | Busy, or limiting how fast you send. Often a rate limit. | Your server retries. Slow down if it's frequent. |
450 | Mailbox unavailable (e.g. busy, or temporarily blocked for policy reasons) | Not right now. Greylisting often uses this. | Let the retry happen. |
451 | Requested action aborted: local error in processing | A problem on their side, or greylisting. | Let the retry happen. |
452 | Insufficient system storage (also used for "too many recipients") | Out of space, or you sent to too many recipients at once. | Retry; send fewer recipients per message. |
550 | Mailbox unavailable (e.g. not found, no access, or rejected for policy reasons) | The catch-all "no". Usually a dead mailbox, sometimes a block. | Read the enhanced code and text. |
551 | User not local; please try another path | That person isn't here. | Remove, or use the new address if given. |
552 | Exceeded storage allocation | Mailbox full. RFC 5321 says to treat it as temporary when it's about recipient count. | Retry later; drop it if it stays full. |
553 | Mailbox name not allowed (e.g. syntax incorrect) | The address itself is invalid. | Remove or correct. |
554 | Transaction failed (or "no SMTP service here" on connect) | A general permanent refusal: often spam filtering or a block. | Read the text. Check reputation and content. |
556 | Domain does not accept mail (RFC 7504) | The domain takes no email at all. | Remove. |
Codes 500 to 504 mean a command was malformed or out of order. They point to a bug in the sending software, not
a problem with the recipient.
Enhanced status codes
The first digit repeats the class (2, 4 or 5), so 4.2.2 and 5.2.2 are the same problem, temporary or permanent.
| Code | RFC meaning | In plain English | What to do |
|---|---|---|---|
5.1.1 | Bad destination mailbox address | No such user. The classic hard bounce. | Remove. |
5.1.2 | Bad destination system address | The domain or host doesn't exist or can't take mail. | Remove; check for a domain typo. |
5.1.3 | Bad destination mailbox address syntax | The address is malformed. | Correct or remove. |
5.1.6 | Destination mailbox has moved, no forwarding address | The person has gone. | Remove. |
5.1.10 | Recipient address has null MX (RFC 7505) | The domain publicly declares it receives no email. | Remove. |
5.2.1 | Mailbox disabled, not accepting messages | Suspended or deactivated account. | Remove. |
4.2.2 / 5.2.2 | Mailbox full | Over quota. Long-full mailboxes are usually abandoned. | Retry; remove if it repeats. |
5.2.3 | Message length exceeds administrative limit | Your message is too big. | Shrink attachments. |
4.4.1 / 4.4.2 | No answer from host / bad connection | Network trouble reaching the server. | Let the retry happen. |
4.4.7 / 5.4.7 | Delivery time expired | Your server gave up after retrying a soft bounce. | Treat as a hard bounce if it keeps happening. |
4.7.0 | Other or undefined security status | A temporary security or reputation hold. Large providers use it for "try again later" rate limits. | Slow down; check authentication and reputation. |
5.7.1 | Delivery not authorized, message refused | Blocked: your IP or domain is listed, the recipient only accepts certain senders, or content was refused. | Read the text. Fix the cause, keep the contact. |
5.7.26 | Multiple authentication checks failed (RFC 7372) | SPF and DKIM didn't pass. | Fix SPF, DKIM and DMARC on your sending domain. |
Microsoft 365 often reports a missing recipient as
550 5.4.1 Recipient address rejected: Access denied, even
though RFC 3463 defines 5.4.1 as "no answer from host". Microsoft documents it as
directory-based edge blocking rejecting an invalid recipient
(checked 2026-10-17). It's another case where the text tells you more than the number.How our verifier reads these codes
During a check we only go as far as RCPT TO and never send a message (how),
so we see the codes a server gives at that stage:
| Server reply | MailExchange result |
|---|---|
| 2xx for the address, 5xx for a random test address | deliverable (or risky: role) |
| 2xx for both | risky: low_deliverability (catch-all) |
| 5xx that rejects the mailbox (5.1.1, "user unknown", "does not exist") | undeliverable: rejected_email |
| 5xx with 5.7.x, blocklist or policy wording, and no "no such user" wording | unknown: blocked_by_server |
| 4xx, still 4xx after a retry a few minutes later | unknown: unavailable_smtp |
552 (mailbox full or over limit) | unknown: unavailable_smtp |
Unknowns are refunded. The full list of results is in email verification statuses.