The SMTP command was recognized but contained invalid parameters. Common causes include malformed email addresses in MAIL FROM or RCPT TO commands, invalid characters, or improperly formatted command arguments.
What Does Error 501 Mean?
SMTP code 501 indicates the receiving server recognized the command but found a syntax error in its parameters or arguments. This most commonly occurs with the MAIL FROM or RCPT TO commands when the email address is malformed, contains invalid characters, or is not properly formatted with angle brackets.
This error can also occur during EHLO when the sending server provides an invalid hostname, or during AUTH when credentials are improperly encoded. Some receiving servers are stricter about RFC compliance than others, so an address that works with one provider might cause a 501 with another.
Common Causes
- Malformed email address in MAIL FROM or RCPT TO (missing @, invalid characters)
- Missing angle brackets around email addresses in SMTP commands
- Invalid or unresolvable hostname in EHLO/HELO command
- Improperly Base64-encoded AUTH credentials
- Non-ASCII characters in addresses without proper SMTPUTF8 support
How to Fix Error 501
- Verify the sender and recipient email addresses are properly formatted
- Ensure your MTA uses angle brackets in MAIL FROM and RCPT TO commands
- Check that your EHLO hostname is a valid, resolvable domain name
- Verify AUTH credentials are properly encoded
- Review your MTA logs for the exact command that triggered the error
Frequently Asked Questions
SMTP error 501 means "Syntax error in parameters or arguments" -- the mail server received a command with incorrectly formatted data. The most common cause is a malformed email address missing the "@" symbol, containing invalid characters, or having an incorrect domain format. Check all email addresses in the To, From, CC, and BCC fields for typos and formatting errors.
To fix SMTP 501, first verify all email addresses follow the standard format (user@domain.com) with no extra spaces, special characters, or missing components. Ensure your SMTP client uses a fully qualified domain name (FQDN) in the EHLO/HELO command. Check that the email subject line and headers do not contain unsupported characters. If using command-line SMTP, confirm the total command length does not exceed 512 characters.
SMTP 501 "bad sender address" is caused by an improperly formatted sender email address in the MAIL FROM command. Common triggers include missing angle brackets around the address (some strict servers require them), an invalid or non-existent domain name, special characters that violate RFC standards, or a Postfix server with "strict_rfc821_envelopes" enabled. Verify your sender address configuration in your email client or application settings.