I'm using Moodle 2.5, and I've been trying to configure the email settings. The site has never sent out a single email, not for forum post, new user notifications, or anything else, even when we first set the site up with all the defaults. I have forced subscription set up on the forums, I hit the "mail now" option, and still nothing goes out to those I reply to. I've tried it with the default Moodle settings, with my gmail account, and I even set up a yahoo account to see if that would work (it didn't). I turned on debugging, and have it set to show all email errors, but I get nothing in my logs. I've searched the forums extensively, but I still haven't found an answer, and I'm at a complete loss.
Sounds like you are on a remotely hosted (shared?) system? Check the package you've purchased for things related to EMail. Some remotely hosted shared systems have very strict policy on Email distributions (they don't want their systems to be used by spammers). Check providers FAQ's/Knowledge Base or whatever they provide for customers to learn about the systems they are using.
If you have access to any logs (apache or email), search those for a clue.
'spirit of sharing', Ken