What it looks like in your logs
553 5.7.1 <newsletter@example.com>: Sender address rejected: not owned by user sender@example.orgWhat does 553 mean?
RFC 5321 defines 553 as a requested action not taken because the mailbox name is not allowed, with incorrect syntax as the classic example. Unlike 550, it is not saying the mailbox does not exist. It is saying the server will not accept that address in that position, whether because of its format or because of a policy. Your server will not retry it, so something has to change before a resend can work.
The address in question can be the recipient or the sender. Many relay servers return 553 when the MAIL FROM address does not belong to the authenticated account, for example when an app logs in as one user and tries to send as another. Others use 553 for relay attempts, meaning you asked the server to deliver to a domain it does not handle and you are not authorized to relay through it.
Postfix commonly returns "553 5.7.1 Sender address rejected: not owned by user". Older qmail servers answer "553 sorry, that domain isn't in my list of allowed rcpthosts" for relay attempts. Outlook desktop users usually meet 553 when their SMTP server settings or sending identity do not match the account.
Soft or hard bounce?
| Soft bounce (4xx) | Hard bounce (5xx) | |
|---|---|---|
| Nature | Temporary | Permanent |
| Sender behaviour | Queue and retry with backoff | Stop, suppress the address |
| Reputation risk | Low, unless it repeats for days | High above a 2% bounce rate |
| 553 | ✓ this code |
Common causes
- The recipient address has invalid syntax, stray spaces or characters the server rejects.
- The sender address in MAIL FROM is not owned by the authenticated account.
- You tried to relay through a server that does not handle the recipient domain.
- Your client is not authenticating, so the server treats the send as an open relay attempt.
- The server blocks certain mailbox names or patterns by policy.
How to fix 553
- Check whether the bounce names the sender or the recipient address.
- For recipient syntax errors, clean the address and validate it at the point of capture.
- For sender rejections, send from an address the authenticated account is allowed to use, or add it as an alias.
- Turn on SMTP authentication in your client and use the correct outgoing server for your account.
- Confirm the recipient domain's mail servers with the MX lookup if you suspect you are connecting to the wrong host.
553 questions
What does SMTP error 553 mean?
It means the server refused an address as not allowed. The address may be malformed, it may be a sender your account is not permitted to use, or you may be trying to relay mail through a server that does not accept it. Read the text to see which address was rejected.
What does "553 relay not permitted" mean?
It means you asked a server to deliver mail to a domain it does not host, and you are not authenticated or authorized to use it as a relay. Enable SMTP authentication in your client, or send through the outgoing server your provider assigned to your account.
How do I fix error 553 in Outlook?
Open the account settings and check the outgoing server. Make sure "My outgoing server requires authentication" is enabled, the port is 587 with STARTTLS, and the From address matches the account or an approved alias. A mismatched sending identity is the most common cause of 553 in desktop clients.
Is 553 the same as 550?
No. A 550 usually means the mailbox does not exist or a policy blocked you. A 553 means the address itself is not acceptable in that transaction, because of its format or because you are not allowed to use it. Both are permanent.