Instead I have used a PHP class that allows me to send emails using a remote smtp server using an account on that server. This has been a good solution for my setup anyways. A few days ago a friend of mine was asked to investigate the very same problem for a client.

As it turns out, the solution to their problem was pretty simple - a conflict between the sendmail_from in the php.ini and the "From" passed into the mail function call. A simple ini_set resolved the issue and kept the spam filters from catching and blocking the message.