This is what the email header would look like:

X-Authentication-Warning: mail.domain.com: apache set sender to sales@domain.com using -f

To keep sendmail from adding the warning, you need to setup your apache user as a trusted sender. In my case my apache user is "apache". Sometimes the user might be called httpd.

You will need to add your apache user to /etc/mail/trusted-users

[server][root][~]# vi /etc/mail/trusted-users

Your sendmail.cf should be ready for that:

[server][root][~]# grep trusted /etc/mail/sendmail.cf
Ft/etc/mail/trusted-users

Otherwise force the trusted user with a line like:

Tusername

If you build your sendmail.cf from sendmail.mc, use:

FEATURE(use_ct_file)dnl