You deleted a user account/mailbox, you panic!
Before you go to your backups... lets just check that we have mailbox retentions enabled.
Server Configuration --> Mailbox --> Right click mailbox database (not storage group) hit properties --> Retention is under Deleted Settings on the Limits Tab.
User When a mailbox user/mailbox is deleted, it does not get removed from the exchange server until the retention has expired. However it does not get marked as a disconnected mailbox until the mailbox cleanup agent runs. The mailbox cleanup agent runs nightly by default as scheduled.
Note: Disconnected mailboxes are mailboxes marked as deleted pending their awaited retention period to expire before being permanently removed.
Scenario: You accidently delete a user account with a mailbox attached. Well if you have two DC's, great, reboot the one into directory services restore mode, restore sysstate then fireup your ntdsutil to mark the deleted user account as authorative. Back in business. But what if you only have one DC?
An alternative method is to create a new user account for that user, put him in the appropriate groups, then link his deleted mailbox to that account. You fire up your exchange management console/shell look in the disconnected users area, but hey, its not in the disconnected mailboxes area. Bet your panicing now. The reason its not in the disconnected mailboxes area is because exchange only realises a mailbox has been disconnected/deleted once the cleanup agent runs, scheduled nightly by default. Yes you can kick it off manually in management shell. Fire up your exchange management shell. The command you want is:
Clean-MailboxDatabase "Mailbox Database"
You have 30 mailbox databases, how do you know which mailbox database the mailbox was in? Who cares if you dont know, just run it on all by piping the Get-MailboxDatabase command.
Your new user account is back up online, however ppl's emails are bouncing saying recipient object cannot be found. Yes this is a pain in the ass. This is because the GUID id of the new mailbox does not match that of the NK2 file on the client workstations. The outlook.NK2 holds all the autocomplete contacts for outlook. The NK2 populates itself from the offline address book on a clients workstation. For affected users, you need to update the offline address book as it updates every 24 hours by default, then delete the NK2 file. Google for more info.