SMTP Error 553

SMTP reply 553 is a permanent negative completion reply. RFC 5321 §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 a MAIL FROM: or RCPT TO: command. The receiving (or your outbound) server is telling you it will not accept the address you supplied. Because it is a 5xx code, it is a hard failure — retrying the same message unchanged will not help.

What causes SMTP Error 553?

There are two distinct root causes, and the wording after 553 tells you which one you hit:

1. The mailbox/address is syntactically invalid (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 passed where a bare address belongs. The SMTP envelope expects the bare address user@example.com — passing "John Doe" <user@example.com> into RCPT TO can trip this.

2. The sender address or relay is not permitted (the most common real-world case). Here the address is well-formed, but the server refuses to send from or relay through it. Typical wordings:

• 553 Sender address rejected: not owned by user — Postfix reject_sender_login_mismatch: you authenticated as one account but set MAIL FROM to an address that account doesn't own.
• 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1) — you tried to relay to an external domain without authenticating. This exact string is documented in Microsoft's guidance on 550/553/relay-prohibited errors. Note: some servers phrase the same underlying cause (unauthenticated relay) as 550 ... Relaying Denied instead — same root cause, different code.
• 553 5.1.8 ... MAIL-FROM domain does not exist in DNS — the envelope-from domain has no resolvable A/AAAA/MX record.

Note: some senders see 553 codes carrying enhanced status codes (RFC 3463) like 5.1.x (addressing) or 5.7.1 (policy). The enhanced code is the most precise diagnostic — read it before acting.

How do I fix SMTP Error 553?

Work the cause that matches your error text:

If the address is malformed (syntax):

1. Validate the exact RCPT TO / MAIL FROM value — not the display name. Confirm one @, a real TLD, and no leading/trailing whitespace.
2. Strip hidden characters. Re-type the address by hand rather than pasting; addresses copied from spreadsheets or rich-text fields often carry zero-width or non-breaking spaces.
3. Pass only the bare address into the envelope. Keep "John Doe" <…> formatting in the To: header, never in the SMTP command.

If the sender/relay is rejected:

1. Authenticate. Configure SMTP AUTH with valid credentials. For Gmail/Google Workspace, use an App Password (requires 2-Step Verification) or OAuth2. Google has phased out password-only "Less Secure Apps" access — personal accounts since May 2022, and Google Workspace fully by May 2025 — so that toggle no longer exists on either. Use port 587 (STARTTLS) or 465 (implicit TLS); both are valid per RFC 8314.
2. 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.
3. Verify envelope-from DNS if you see 5.1.8 — ensure the MAIL-FROM domain resolves (A/AAAA or MX).
4. Authenticate your domain (SPF, DKIM, DMARC) so downstream servers accept the sender.