Why does the autoresponder sender show as being sent on behalf of the email user?

Question

Why does the autoresponder sender show as being sent on behalf of the email user?

cpaneluser@hostname.server.tld on behalf of emailuser@domain.tld

Answer

Autoresponder messages are sent by the autorespond script. Scripts must be run by a system user, not an email user, and all messages sent via a script will show the authenticated user as the system user who ran the script. The system account the autorespond script runs as is cPanel user for the cPanel account the email account is on. Because of this, the authenticated user (Sender header) is the cPanel user at the server's hostname (cpaneluser@hostname.server.tld), and the From, envelope-from, and Return-Path headers use the email account's email address. Many email clients will show the sender as the email address in the From header, while others, such as Microsoft Outlook, will display the sender as the authenticated system user on behalf of the unauthenticated email account.

Additional resources

Autoresponders