Your first error would be due to backup.pst file which you're trying to backup. User1 obviously has a backup of their data in a pst file on that server, however when the backup was done the pst file was in use, presumably because the users copy of outlook was open at the time, and it had the pst file open.
This is fairly common, I think there is something about the way that pst files get locked when in use which makes backup exec think they are corrupt (you get the same thing with quickbooks databases). Not sure there is a lot you can do other than ensuring the user doesn't have the pst open when the backup is run.
Not sure about the other two errors though. the CRC error on your second one could be multiple things, normally relating to either the drive hardware or a problem with the tape. If you search the knowledge base there are loads of articles relating to the various things which can cause that error.
If if the third error was relating to a normal folder on the server I would guess that it was due to the folder being removed, but the selection still being there in the backup properties, but since it is an Exchange folder I would assume this is not the case, since you Exchange server wouldn't be working, and at a guess I figure you would have heard about that if that had been the case! :) So I'm not sure what it is relating to in this instance.
