Enhanced SMTP status code · RFC 3463 · IANA

5.5.1 Invalid command

An SMTP command was sent out of sequence or is not supported.

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.
Associated basic codes (IANA registry)
500501503530550554555
Source
RFC 3463 · IANA

How 5.5.1 appears at providers

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

Gmail

503 5.5.1Bad sequence of commands.

The SMTP client sent a command that is unknown, not implemented or out of order (RFC 5321). The problem is in the sending software.
502 5.5.1Unimplemented command.

The SMTP client sent a command that is unknown, not implemented or out of order (RFC 5321). The problem is in the sending software.
502 5.5.1Unrecognized command.

The SMTP client sent a command that is unknown, not implemented or out of order (RFC 5321). The problem is in the sending software.
502 5.5.1Too many unrecognized commands, goodbye.

The SMTP client sent a command that is unknown, not implemented or out of order (RFC 5321). The problem is in the sending software.
503 5.5.1No DATA after BDAT. An email transaction protocol command was issued out of sequence.

The SMTP client sent a command that is unknown, not implemented or out of order (RFC 5321). The problem is in the sending software.
503 5.5.1EHLO/HELO first. An email transaction protocol command was issued out of sequence.

The SMTP client sent a command that is unknown, not implemented or out of order (RFC 5321). The problem is in the sending software.
503 5.5.1An email transaction protocol command was issued out of sequence.

The SMTP client sent a command that is unknown, not implemented or out of order (RFC 5321). The problem is in the sending software.
503 5.5.1RCPT first. An email transaction protocol command was issued out of sequence.

The SMTP client sent a command that is unknown, not implemented or out of order (RFC 5321). The problem is in the sending software.

Have the whole error message?

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