Issue: Exchange is unable to mount the database that you specified. Specified database: ServerEXVS25SG1Mailbox; Error code: MapiExceptionCallFailed: Unable to mount database. (hr=0x80004005, ec=-2147467259)
Resolution:
Error was due to lag in AD replication. Whenever a new mailstore is created, it updates the config in AD. Trying to mount too soon may reproduce the error stated above. So wait about 5 to 10mins and try to mount it again. It should mount OK
Ex-Shell 