In a new post on the Dotvoid.com blog today, Danne Lundqvist talks about some of the issues he's had with the mail function in PHP. Specifically, it's about the mails being set but not making it to their destinations.
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.