How to undelete/restore a mailbox in Archive Manager using the AM Admin web UI?
It is not currently possible to undelete a mailbox via the Archive Manager Administration web site. The mailbox can be restored using a SQL query run against the Archive Manager database, however.
Enhancement Request ST61995 has been raised for this feature to be included in a future version of Archive Manager
Run the below query on the SQL server:
Use Archive Manager
update mailbox set deleted=0 where name ='Firstname, Lastname'