Ok. Another thing I found weird, I uploaded a simple mail test script and all the mail was sent fine using PHP. I'm pressing the MyBB team about this, and I'll post back if their position changes. Thanks for the help.
Edit:
*Achoo*...sorry. If it's time to give this up, just tell me. But I think the fact that a test script can send email leans towards a MyBB problem.
Edit:
I don't get this! How come everyone has given up on this? Even the MyBB team couldn't help me. Is it just that my account is not allowed to send email when I'm using MyBB or something? My last 2 CMS's sent email just fine, but I can't get MyBB to send any email using either PHP mail() or any SMTP server. How come no one else has this problem with MyBB?
Edit:
OK, completely forget this. I must be going crazy, because all of a sudden me emails started getting received, and in my inbox just appeared every email I tried to send for the past 3 days with the correct date and timestamps. Sorry for wasting everyone's time.