SMTP error 553
Mailbox name not allowed / invalid address.
Updated Jul 1, 2026
SMTP 553 is a permanent 5xx reply defined in RFC 5321 as "requested action not taken: mailbox name not allowed", returned when the server rejects the address in MAIL FROM or RCPT TO. In practice it means either the address is syntactically invalid, or the server will not let you send from or relay through it. Fix it by correcting the address syntax and authenticating with a sender address you are allowed to use.
Check the address for typos and formatting errors.
What it means
553 is a permanent negative completion reply. RFC 5321 section 4.2.3 defines it as "553 Requested action not taken: mailbox name not allowed (e.g., mailbox syntax incorrect)". It is returned in response to MAIL FROM or RCPT TO. The receiving or outbound server is telling you it will not accept the address you supplied, and because it is a 5xx code it is a hard failure, so retrying the same message unchanged will not help. The wording after 553 tells you which of two root causes you hit.
Common causes
The literal RFC meaning. The address in MAIL FROM or RCPT TO is malformed: a missing or extra @, a missing top-level domain, illegal or hidden characters (zero-width or non-breaking spaces pasted from a CRM), or a display-name string where a bare address belongs. The envelope expects the bare address user@example.com, so passing "John Doe" <user@example.com> into RCPT TO can trip this.
"553 Sender address rejected: not owned by user" is Postfix reject_sender_login_mismatch: you authenticated as one account but set MAIL FROM to an address that account does not own.
"553 sorry, that domain is not in my list of allowed rcpthosts (#5.7.1)" means you tried to relay to an external domain without authenticating. Some servers phrase the same cause as "550 ... Relaying Denied" instead, same root cause, different code.
"553 5.1.8 ... MAIL-FROM domain does not exist in DNS" means the envelope-from domain has no resolvable A, AAAA, or MX record.
How to fix it
- If the address is malformed, fix the syntax
Validate the exact RCPT TO and MAIL FROM value, not the display name: one @, a real TLD, and no leading or trailing whitespace. Strip hidden characters by re-typing the address by hand rather than pasting, since addresses copied from spreadsheets or rich-text fields often carry zero-width or non-breaking spaces. Pass only the bare address into the envelope and keep any "John Doe" <...> formatting in the To header, never in the SMTP command.
- If the sender or relay is rejected, authenticate
Configure SMTP AUTH with valid credentials. For Gmail or Google Workspace, use an App Password with 2-Step Verification or OAuth2, since password-only Less Secure Apps access is gone (personal accounts since May 2022, Workspace fully by May 2025). Use port 587 with STARTTLS or 465 with implicit TLS, both valid per RFC 8314.
- Match the From to an address you own
Set MAIL FROM and the visible From to an address the authenticated account is authorized to send as. With Postfix smtpd_sender_login_maps, the login must be permitted for that sender.
- Verify envelope-from DNS for a 5.1.8
Ensure the MAIL-FROM domain resolves with an A, AAAA, or MX record.
- Authenticate your domain
Publish SPF, DKIM, and DMARC so downstream servers accept the sender.
Paired code
A real bounce often shows both a 3-digit code and an enhanced code together. This one commonly pairs with:
Common questions
Is SMTP Error 553 the same as 550?
No. Both are permanent 5xx failures, but 550 generally means the mailbox is unavailable or the message was rejected by policy, while 553 specifically means the mailbox name or address was not allowed, usually invalid syntax (RFC 5321 section 4.2.3) or a sender or relay that is not permitted.
Does retrying fix a 553 error?
No. 553 is permanent, so the same message unchanged fails again. Correct the address syntax, or authenticate with a sender address you are allowed to use, before resending.
What does "553 Sender address rejected: not owned by user" mean?
You authenticated as one account but set MAIL FROM to an address that account does not own. Set the sender to an address the authenticated login is authorized to use, which in Postfix is governed by smtpd_sender_login_maps.