After updating forums from 3.0.1 all the way to 3.0.5. I found out that some emails weren't being sent per an error in the error log.
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E-mail error
?? EMAIL/PHP/mail()
/forums/posting.php

[phpBB Debug] PHP Notice: in file /includes/functions_messenger.php on line 446: mail(): Bad parameters to mail() function, mail not sent.
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I narrowed down the error to when there was an email to be sent as a notification for a new post in a followed topic.

I posted a support forums: http://www.phpbb.com/community/viewtopic.php?f=46&t=1635645

After enough trial and error another user came up with the solution to swap out the .../includes/functions_messenger.php from the v.3.0.4 update with the v.3.0.5 one and it worked.
Multiple people have confirmed this to be a temporary solution.