PHP mail() is very delayed or does not send.

[briman9427]

I have a script on my site that takes POST data from a form and sends an email using PHP's mail() function. I have gotten it to work before, and it was almost instantaneous. Recently, however, it has stopped working completely or the email comes through at least 24 hours after it was sent. The function does not return an error, but no email comes through. Is there something I can change to fix this on my cPanel or is this a server problem?