The email address format is invalid or not allowed by the server's naming policies.
SMTP error 553 indicates the mailbox name format is unacceptable:
The email address contains special characters that aren't allowed.
The domain part of the address is malformed or missing.
Extra spaces or whitespace in the email address.
The server's policy doesn't allow this address format or pattern.
Ensure the address follows standard format: username@domain.com 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.
553 5.1.3 Mailbox name not allowed
553 5.1.2 Bad destination mailbox address syntax
553 Invalid mailbox syntax