Mail sending, picking up many times (but not really)

We are a school running about 100 Macbook Pro laptops running OS 10.7.5. For a couple of our users we've had the experience that their Mail program seems to go a little bit crazy and makes a whole bunch of picking up (bong) and sending (whoosh) sounds when they haven't chosen to send anything. Looking in the in and out boxes shows that nothing has been sent or picked up. Quitting Mail and reopening it stops the flow of sounds. The laptops have checked out as virus-free with Sophos. It's disconcerting because we have been seeing an increase in spam recently, and specifically with spam claiming to be sent by these two users and received by others staff in our school, but again, nothing in the user's out box.

The many sounds problem has also been reported in the OS 10.6 thread at https://discussions.apple.com/thread/3708338?start=60&tstart=0 under the title "I sometimes get the sound of outgoing mail when nothing was sent". I've read the entirety of that thread but the solutions mentioned don't work / apply here.

Has anyone else found a cause for this?