Ok, I've just tested it, and it looks like it might work as planned ! The real test with all users will happen in 3 months earliest, but what I've seen so far looks promising!

I just created a 2nd MAPI-session before the first one is finished. That way, the user does not need to enter username and password again, since there is already an open connection . The 2nd session logs in with forced download, so I can see the download window for a millisecond, and the mail is really sent not just sitting in the outbox.
Now I close first the 2nd session, then the original one and everybody is happy again!

Thanks again for the tip using the FORCE_DOWNLOAD flag!!

Bye
Daniel