Here we go:
System: Microsoft Exchange 2k7
Client OS: Win XP
Client Mail: Outlook 2k3
Issue: one user (so far) cannot receive mails only from certain INTERNAL users both on his Outlook and Web Access.
Checked the Message tracking Results: ALL missing messages appear in the Message Tracking Results, and they are from different sources: SDMT and STOREDRIVER
User doesn't have any filter applied in his ADUC or mailbox;
Reset the users' profile. Result: nothing