503 Valid RCPT command must precede DATA

Symptoms

When attempting to send an email message, you receive an error similar to the following.

503 Valid RCPT command must precede DATA errors

Description

The error commonly occurs when the sending domain isn't in the /etc/localdomains file.

Workaround

Change the email routing for the domain from remote to local.  The email routing can be changed in the user's cPanel account or WHM.

Additional resources

How to change the MX or Mail Routing for a domain in WHM

How to change the email routing of a domain in cPanel