SMTP code 501 means the server understood the command verb (MAIL, RCPT, EHLO, etc.) but the parameters that followed it are syntactically invalid. This is more specific than 500 (unrecognized command) — the command itself is fine but its arguments are wrong. Common triggers include an invalid email address format in MAIL FROM or RCPT TO (missing angle brackets, illegal characters, malformed domain), an invalid EHLO hostname, or unsupported ESMTP parameters. The client should fix the parameters and reissue the command.
The email address does not conform to RFC 5321 syntax. Common issues include missing angle brackets (should be MAIL FROM:<[email protected]>), spaces inside the address, or illegal characters. Some servers are strict about requiring angle brackets even though the RFC allows them to be optional.
The EHLO parameter should be a fully qualified domain name or an IP address in brackets (e.g., EHLO [192.168.1.1]). Sending EHLO with an empty string, a bare IP without brackets, or an invalid hostname triggers 501 on strict servers.
The client included an ESMTP extension in the MAIL FROM command (e.g., SIZE=, BODY=, AUTH=) that the server does not recognize. Check the server's EHLO response for supported extensions.
Non-ASCII characters in the email address without proper SMTPUTF8 support, or incorrect base64 encoding in AUTH command parameters, can produce 501 errors.
Ensure MAIL FROM and RCPT TO use the correct syntax with angle brackets: MAIL FROM:<[email protected]>. Remove any extra spaces or special characters.
# Correct format: MAIL FROM:<[email protected]> RCPT TO:<[email protected]>
Send EHLO with your server's FQDN. If you do not have a hostname, use your IP in brackets: EHLO [203.0.113.1].
telnet mail.example.com 25
Review the extensions your client is using in the MAIL FROM command. Only include extensions that the server advertised in its EHLO response (SIZE, 8BITMIME, STARTTLS, etc.).
Turn on SMTP debug logging in your application or mail client to see the exact command and parameters being sent. Compare them against RFC 5321 syntax requirements.
The server did not recognize the SMTP command. The command may be misspelled, malformed, or unsupported.
The SMTP commands were sent in the wrong order. The server requires a specific command sequence.
The email address syntax is invalid or the mailbox name violates server naming rules.