After restoring an Exchange 2010 database to a recovery database you can use the following command to recover the contents of the mailbox in the recovery database to an alternate mailbox on a production Exchange 2010 DB
New-MailboxRestoreRequest -SourceDatabase rdb -SourceStoreMailbox “Mailboxtoberestored” -TargetMailbox mailboxtorestoreto -AllowLegacyDNMismatch
-SourceDatabase is the name of the recovery database you restored from backup
-SourceStoreMailbox is the name of the mailbox in the recovery database you restored from backup
-TargetMailbox is the name of the mailbox you want to restore to, this woulkd reside on one of your production Exchange databases
-AllowLegacyDNMismatch used when the source mailbox has a different legacyexchangedn than the target mailbox