found the bu99er!
It was f$%^king AVG causing the problems with some emails. Having un-installed Mailwasher, ZoneAlarm and a few other things I finally got to look at AVG which was proudly telling me that the system was "fully protected". I disabled the mail checking in AVG, sent a sample mail from Yahoo, and it came through intact.
Maybe time to try another anti virus, any suggestions?
Perhaps Avast............
Rans6....