Basic SMTP reply code · RFC 5321

503 Bad sequence of commands

The command came out of order, for example RCPT before MAIL FROM.

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:

5.5.05.5.15.5.45.7.05.7.1
Source
RFC 5321

How 503 appears at providers

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

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.
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.
503 5.7.0No identity changes permitted.

The client tried to change identity within the same session, which Gmail does not allow.

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