SMTP Error 501: Syntax Error in Parameters or Arguments
SMTP Error 501 means “Syntax Error in Parameters or Arguments.” 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.
501 5.1.3 Bad recipient address syntax <john.doe @example.com>What does 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.
How 501 plays out
501 rejectionWhere 501 sits: soft vs hard bounce
| Soft bounce (4xx) | Hard bounce (5xx) | |
|---|---|---|
| Nature | Temporary | Permanent |
| SMTP class | 4xx | 5xx |
| What to do | Let it retry | Suppress the address |
| Recoverable? | Often | No |
| 501 is | ✓ this code |
Common causes of 501
- 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 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