Basic SMTP reply code · RFC 5321

501 Syntax error in parameters

Syntax error in the command's parameters or arguments.

Class: 5yz · Permanent failure
5yz: a permanent failure. The same request should not be repeated; something must change first, such as the address (RFC 5321 §4.2.1).
Associated enhanced codes
The enhanced codes that usually come with this code, per the registry and the providers:

5.1.35.1.85.5.05.5.15.5.25.5.45.7.05.7.11
Source
RFC 5321

How 501 appears at providers

The exact text from the provider's documentation, as you see it in the bounce, and what it means.

501 5.5.2Syntax error, cannot decode response.

The SMTP command sent could not be understood (a syntax error).
501 5.5.4HELO/EHLO argument argument invalid closing connection.

The HELO/EHLO name is invalid or missing. The server must introduce itself with its full name. Check with Mail delivery test.
501 5.5.4Empty HELO/EHLO argument not allowed, closing connection.

The HELO/EHLO name is invalid or missing. The server must introduce itself with its full name. Check with Mail delivery test.
501 5.7.11Syntax error (no parameters allowed).

STARTTLS was sent with parameters, and RFC 3207 allows none.

Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026