Change the FID of the old user's archive user.db so it is the same as an existing user, usually an IT Admin. Then when the IT Admin wants to access that old user, simply change the Archive path to where those files are, usually on a different drive.
This is also an excellent way to move all email from one user to another, while maintaining cabinet folder structures.
To edit the FID on an existing archive to be opened by a user with a different FID you may use the following tools:
Download the Archive FID editor from Novell, gwafe510.exe
Then Read TID 10014261
Just make sure you get the syntax correct and the actual change takes 5 seconds.