Is there any way to set the retention period for deleted mailboxes on a single mailbox or can this value only be set at mailbox DB level?

"retention period for deleted mailboxes" on a "single mailbox"? Are you sure that's what you mean? "Deleted mailboxes" are when an administrator marks a mailbox for deletion and it's held on the database for 30 days by default. Perhaps you mean "deleted item retention" whereby items that a user deletes are held for X days?Neil Hobson, Exchange MVP

That's what I mean alright. Delete the entire mailbox from the database.

Not that I know of but I'm happy to be wrong. There is the -MailboxRetention parameter of the Set-MailboxDatabase cmdlet, but I don't see a similar option for the Set-Mailbox cmdlet.Neil Hobson, Exchange MVP

that's what i was thinking too ;-)