As a follow-up to my post on getting rid of the X-Authentication-Warning error when using phpList on FreeBSD, here’s how to do the same in Linux.

Open up /etc/mail/trusted-users in your favorite editor, and add both the user that your Apache is running under, as well as any usernames which are processing phpList queues and bounces via crontab. For example, your trusted-users file should look like this:
httpd
yourusername
Save & Exit. Restart sendmail with the following command:
/etc/init.d/sendmail restart
Now when you send mail with phpList, the headers won’t contain X-Authentication-Warnings.

One security drawback that you should be aware of: Any user on your system can now use PHP to send email with “forged” headers. You basically just gave everyone on your system “Trusted User” status to Sendmail, so be sure that you trust your users before actually doing this!

See how to do this on FreeBSD