How To: Restore a removed Mailbox Database

After a Mailbox Database is deleted, the actual database file (.edb) is still there. To recreate the original database from this file, do the following:

1. Create a new empty mailbox database, using the New-MailboxDatabase command
2. Enable "Over-written by Restore", using Set-MailBoxDatabase
3. Move the old database files to the same location as the new database
4. Mount the database
5. If restoring on a new exchange server, run Move-Mailbox - ConfigurationOnly

see http://technet.microsoft.com/en-us/library/aa998340.aspx