SMTP Error 553 - Invalid Mailbox Name / Syntax

What does this error mean?

SMTP error 553 indicates the mailbox name format is unacceptable:

alternate_email

Invalid Characters

The email address contains special characters that aren't allowed.

language

Bad Domain Format

The domain part of the address is malformed or missing.

space_bar

Spacing Issues

Extra spaces or whitespace in the email address.

policy

Policy Restriction

The server's policy doesn't allow this address format or pattern.

Ensure the address follows standard format: [email protected] with no spaces or unusual characters.

Avoid characters like: spaces, quotes, parentheses, brackets, and most symbols except . - _ +

Make sure the domain after @ is complete and correct (e.g., gmail.com not gmail).

If the address looks valid, contact the recipient's mail administrator about their naming policies.

Example response

553 5.1.3 Mailbox name not allowed
553 5.1.2 Bad destination mailbox address syntax
553 Invalid mailbox syntax