When you have refresh a forest and the majority of mailboxes have appeared for an On Premise exchange setup but there is one or only a few mailboxes missing.
原因
You have confirmed that the missing object is actually a mailbox but it still does not appear - this means we are not attempting to read that mailbox in.
対策
Confirm that the database that the missing mailbox resides in is part of the selection. We read in based upon the WhenChanged date - make a change to the mailbox so the WhenChanged date updates and refresh the forest.