To anyone who has found this post because you are having the same issue I am:
I have stumbled upon another moodle thread that might hold one possible answer to this problem:
http://moodle.org/mod/forum/discuss.php?d=20318#98490
It sounds exactly like my predicament and so I have asked my web host and there is, indeed, a limit placed on the number of emails that can be sent out in a 10 second period! This is causing the forum mailer errors. So far my web host is offering A LOT of resistance to removing the limit. I keep sending them pleading emails and so I hope they help me. I don't know what to do if they refuse... 