1) Assuming you have deleted the user from Active Directory, either recreate the user or create a temp user
2) In Exchange Management Console > Recipient Configuration > Disconnected Mailbox, right click on the mailbox you wish to reconnect > connect
3) In the ‘Connect Mailbox’ wizard, select User Mailbox.
Select ‘Existing User’ and choose the recreated or temp user from step 1
4) Log into Outlook as the user from step 1.
If Outlook errors, this is because replication hasn’t fully occurred. This can be resolved by:
a) In Active Directory Sites and Services, replicate between DC’s (see below)
Attempt to log into Outlook again.