SMTP error 550, what it means and what to do

SMTP 550 is the most common permanent mail rejection (RFC 5321: "Requested action not taken: mailbox unavailable"). Permanent is the key word: the receiving server will not accept this message as addressed or from this sender, and retrying unchanged achieves nothing. The words and enhanced code after 550 split it into distinct problems: 5.1.1 means the recipient does not exist at that domain, 5.7.1 family means policy (relay denied, sender blocked, authentication of the domain failed), and a bare 550 with a local reason string follows whatever that text says. Read the full line before acting.

The fix, in order

  1. Read the whole response, not just 550. The text after the code (for example "5.1.1 User unknown", "Relaying denied", or "SPF check failed") is the actual diagnosis. Everything below is chosen based on that sentence.
  2. If it says user unknown or no such mailbox (5.1.1): the recipient address is wrong or the mailbox is gone. Check spelling, confirm the address with the recipient by another channel, and remove dead addresses from mailing lists. Nothing on the sending server can fix a nonexistent recipient.
  3. If it says relay denied or not permitted (5.7.1): the message was sent through a server that is not allowed to relay for the sender. In a mail client, enable SMTP authentication and send through the provider's submission port 587 instead of an unauthenticated connection; the server then knows who is sending and relays legitimately.
  4. If it names SPF, DKIM, or sender policy: the sending domain's DNS records are the fault. The domain needs a valid SPF record authorizing the sending server, and ideally DKIM signing and a DMARC record. These are set by whoever manages the domain's DNS, not in the mail client.
  5. If it says the sender or IP is blocked: the sending server's address has landed on the receiving side's blocklist. Check the IP against public blocklists, and if it is listed, follow that list's removal process after fixing the cause (usually a compromised account or a misconfigured server sending spam-like traffic).
  6. Test with a plain message to a known address. A short text-only message to an address at a large provider separates content problems (links, attachments, formatting that trip filters) from identity problems (addressing, relay, DNS). Which one fails tells the owner of the fix.

If it still happens

The sender-versus-server split for 550: addressing faults (5.1.1) belong to the sender's address book; relay and policy faults (5.7.x) belong to the sending setup, either the client's authentication settings or the domain's DNS and the server's reputation. A 550 is the receiving server's final answer for that message, so change something real before resending.

Common questions

Is SMTP 550 temporary or permanent? Permanent, in the 5xx sense: the server has refused this message and will refuse identical retries. Mail servers do not queue and retry after a 550 the way they do after a 4xx code.

Can a 550 mean the recipient's mailbox is full? Not usually. Full mailboxes produce 552 or a 452/550 variant whose text explicitly mentions quota or storage. When the text says mailbox unavailable with 5.1.1, it means the mailbox does not exist, which is a different problem.

Related errors

SMTP error 554 · SMTP error 553 · SMTP error 530