← Error library

503 5.5.1 Bad sequence of commands

permanent Connection

Meaning

Your mail server sent SMTP commands in the wrong order, causing the receiving server to abort the transaction.

Common causes

  • Sending the DATA command before specifying a sender (MAIL FROM) or recipient (RCPT TO).
  • Attempting to authenticate after the mail transaction has already started.
  • A bug in the custom script or application generating the SMTP connection.

How to fix

  1. Ensure your application follows the standard SMTP sequence: EHLO, AUTH, MAIL FROM, RCPT TO, DATA.
  2. If you are writing a custom SMTP client, review RFC 5321.
  3. Use an established email library instead of manually opening sockets and sending raw SMTP commands.

Example bounce

503 5.5.1 Bad sequence of commands.

FAQ

Why does this happen with my custom script?
SMTP is a stateful protocol. You cannot send a message body until the server has acknowledged the sender and recipient. If your script doesn't wait for responses, it will trigger this error.

Related codes