Well Donut...i'm glad you asked!
Ignore Bohica and his "exchanging Phones" philosophy...utter rubbush and clearly no help at all.
The problem as I see it, is that the mail client is giving away certificates at a whim, instead of making things earn them and as such everything is able to have a piece of the proverbial pie. What are the certificates its handing out? Are they like these -



?
So in conclusion I think the best thing to do is to sit down over a cuppa and let the mail client know that its admirable that it is being so generous with its recognition BUT it is becoming a problem and they need to earn these certificates in future.
Another possiblility is that the mail client is taking bribes! By not asking them for their id (user/pass) and letting them into the "club" for free...Indeed, giving them a certificate of attendance as well...seems a bit suss, maybe install a camera and catch the bugger out?
Express and windows mail are upstanding gentleman and wouldn't support this sort of behaviour as you so rightly stated..mozilla on the other hand is a lying, cheating fuck who'll do anything for a quick buck or two, dont turn your back on that cunt for a second...you've been warned.
Hope that helps,
Chalice
IT Guru