SMTP code 553 means the server rejected the sender or recipient address because the mailbox name does not conform to expected syntax or naming conventions. This is different from 550 (mailbox does not exist) — 553 means the address format itself is invalid. Common triggers include addresses with illegal characters, addresses that fail the server's regex validation, null sender addresses where they are not permitted, or addresses in domains that the server does not accept. The enhanced status code 5.1.3 specifically indicates 'bad destination mailbox address syntax.'
The local part (before @) contains characters that the server does not accept. While RFC 5321 allows quoted strings with special characters, most servers reject addresses with unquoted spaces, parentheses, or other unusual characters.
MAIL FROM:<> (null reverse path) is valid for bounce messages but some servers reject it if they do not recognize the context. Additionally, some servers reject MAIL FROM with only whitespace or missing the domain part.
The server is configured to only accept mail for specific domains. If the recipient's domain is not in the server's list of hosted domains, and it is not configured as a relay, it returns 553.
The server attempted to rewrite or expand the address through its alias system but the result was an invalid address. This can happen with misconfigured virtual alias maps or regex-based rewriting rules.
Ensure the address follows the standard format: [email protected]. Remove any special characters, spaces, or non-ASCII characters from the local part. Use angle brackets in the SMTP command: RCPT TO:<[email protected]>.
Try sending to a simple address on the same domain (e.g., [email protected]) to determine if the 553 is specific to the address format or a domain-level issue.
telnet mail.example.com 25
If 553 is returned for MAIL FROM, verify that your sender address is valid and the domain part has MX or A records. Some servers verify the sender domain can receive bounces.
dig MX sender-domain.com +shortCheck DNS Records
The recipient's mailbox does not exist, or the server has permanently rejected the message due to policy.
The SMTP command was recognized but the parameters or arguments are malformed or invalid.
The recipient is not hosted on this server. The server may suggest a forwarding address.