When importing from UBB.Classic / UBB.Threads you'll likely notice that a lot of Private Messages that you've deleted in the past have "magically" appeared out of "nowhere"... There is a very logical reason behind this behavior.
When you send a Private Message (in UBB.Classic / UBB.Threads), there are two participants, when one participant deletes the topic, it still remains for the other.
When creating the importer for UBB.Classic / UBB.Threads a decision had to be made based on the best logical way to import data, especially since some users would not certain PM's in their inbox because they removed them, and the logic of how to do so in a server friendly manner was near impossible, so it came down to either import everything that is current in the PM system, or import nothing.
Not all PM's will re-appear, only those which exist in the inbox of either (or both) participants, and both participants (regardless of if they deleted the PM or not) will receive the message back in their inbox.