Have you ever looked at your email headers to see that sendmail was attaching an X-Authentication-Warning warning because Apache is the sender of an email as a different user other than the system user "apache"?
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