Oh! I missexpressed myself, I wasn't expecting my device to sync the sent messages sent from the computer (that much I know ;) ).

I am registered to some google groups and whenever someone writes a reply to a thread I get an mail, that works fine with my bb unless it is me who makes the reply to the thread, there, although I receive a mail on my inbox online, the bb does not sync with it for some odd reason. That's why I think the problem is actually with my mail inbox rather than with the bb data service plan nor the bb itself.