If you run your system using the Ed Crowley Never Restore Method(tm), you will likely never have to restore a single mailbox. (No guarantees, but it has worked for me.)
1. Implement deleted item recovery for 30 days or more. Users will be able to restore anything THEMSELVES that they have deleted for at least that long.
2. Don’t delete mailboxes right away. Hide them, remove SMTP addresses, make the departing user’s boss the mailbox owner, and enter the termination date in the Notes field. Then wait at least 30 days.
The sad fact is that for nearly all backup programs, to restore a single mailbox you have to back up a single mailbox. Do this for more than a few mailboxes and your backup becomes very time and tape consuming.