I am forwarding emails, via '.forward', through procmail to a PHP script for one of my system users. Once processed the mail message is no longer needed. How can I configure sendmail to delete email messages after they are forwarded?
You are using the pipe feature of forward right ? Then there wouldn't be a copy of the email message in the inbox. I'm guessing that's what you mean by "delete email messages after they are delivered".
If it's not working check for smrsh. srshm defines programs you may pipe mail through for security reasons. Find sm.bin, and place symlinks for programs you want to use for piping.
Else, run a cron job to zero out the inbox once a day or however often works for you.