553 5.1.3 Permanent Failure

Invalid mailbox name syntax

The receiving server has rejected the address because its local part (before the @) doesn't conform to valid mailbox-naming syntax for that server.

Common causes

Typo introducing an invalid character.
Address copied with extra whitespace or formatting artifacts.
A domain-specific restriction on characters in local parts.

What to do

This should also be caught by syntax validation before an SMTP attempt is ever made — if it's reaching this stage, check your pre-send syntax validation step rather than relying on the receiving server to catch it.

FAQ

Why does this happen after a valid-looking syntax check?

Baseline email syntax rules (RFC 5321/5322) are permissive, allowing characters that many individual mail servers don't actually support — a syntax-valid address under the general spec can still be rejected by a specific server's stricter local rules.

Other permanent failure codes

550 5.1.1

User unknown / mailbox does not exist

550 5.7.1

Message rejected due to policy (spam/content)

551 5.1.6

Mailbox has moved, no forwarding address

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