I had the same problem as Carlotta, using Win XP Pro and Outlook Express. After installing 4.5.529 it looks like the problem has disappeared.
Uneducated guess on which settings could be involved (at least these are settings I've checked:
Outlook/Exchange - Inbound mail - Discard it unconditionally
Outlook/Exchange - Advanced - Silent mode
Outlook/Exchange - Heuristics - Silent mode