Enhanced SMTP status code · RFC 3463 · IANA
5.5.4 Invalid command arguments
A valid command was given arguments out of range or not recognised.
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1). The registry says this code is only useful as a permanent failure.
- Subject: X.5 · Protocol
Failures of the delivery protocol: implementation errors or an unreliable connection.
How 5.5.4 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Gmail
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501 5.5.4
HELO/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.
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501 5.5.4
Empty 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.
What you can check here
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026