My WPMU + Buddypress doesn't send notification email anymore

Greetings,

I suddenly have a very big problem with my WPMU+Buddypress install :

My system doesn't send an email anymore when a new user try to registrer. He doesn't get the activation email, and I (as admin) doesn't get the notifcation email.

I did a few mods on my WPMU (like erasing old test users and website and using New Blog Templates) but I don't see anything related.

Also, I'm using "Anti-Splog" and thought it could block the email (since I'm in a big testing phase) so I tried to deactivate it, but it doesn't change.

How can I have my system send the emails back ? It's a big problem since without the confirmation email I cannot create test sites anymore, and I have a big demo planned for tomorro.

Hope you guys can help me soon, thanks in advance !

DJ