Weird Problem PHP Mail Newsletter

[VoidingNixx]

So I ran into this problem today while trying to mail out a newsletter.  They were not mailing.  Nothing in the error logs and the action was completed successfully, but they were not being delivered.  I was using PHP Mail on the forum.

I fixed this by changing over to use SMTP settings... no biggie.  Now, this is where I am becoming mind-boggled.  The server DOES email people when they create new accounts for account verification.  The forum has always been setup to use PHP Mail and the new user emails have always been sent out.  Does sending a Newsletter use some other mail settings somewhere?  I do not understand why the forum will mail new uers... heck, it even emails me when new posts are made on threads I am watching... but the forum doesn't email out a Newsletter.

Anyways, one of these weird problems beyond my understanding...  I just hope that changing it to SMTP doesn't stop some other forum function somewhere hah.

If anyone can shed some light on this, why the forum would email fine in certain areas and not send emails using other forum options, I'd be happy to learn what is happening here.

**edit running SMF 2.0.12