This will be the goto to help solve e-mail delivery issues and talk about many practical issues that happen between developers, admins and scripts that send e-mail and do things that may not be acceptable or cause deliverability problems.
Sendmail Stuff
Edit /etc/mail/sendmail.mc
The problem is that if you send directly out from the server using the mail function, the Return-path of the e-mails will be username@thehostnameoftheserver.com. Let........