Very ocassionally, you might get messages getting “crossed,” but this is extremely rare. If you have an ongoing issue, then you need to do some digging:
1) Is user2 (the former employee) listed anywhere in any of your Address Book replicas? Don’t forget groups which are not autogenerated and possible listings as a forwarding address or alias username in another person doc (like user1’s).
2) Is there any relationship between user1 and user2? Do they have similar names, are they former co-workers, boss and admin or anything like that?
3) Is your system set up to look for “close matches” or “best guesses” for an ambiguous name, or will it only accept mail that matches an existing internal listing? If so, for some reason (possibly relating to my question 2), it could be seeing “user1″ as the best available match for “user2.”
These are good starting points. In general, Domino (or any system) should not be accepting emails for names it doesn’t recognize. The question then becomes, where is the entry that is causing this name recognition?