If a mailbox gets migrated or disabled (= disconnected), the mailbox is not removed from the mailbox database right away. Instead, Exchange uses the retention configuration of the mailbox database in which the mailbox was stored. This is useful if a restore of the mailbox is needed (e.g. in case of accidental deletion).

However, sometimes it may not be necessary to have the mailbox available for undelete. E.g. when you have migrated a large number of mailboxes, you may not wish to keep the old mailbox copies to free up disk space. In that case, you can manually "purge" the mailbox from the mailbox database.

Note that this only works with a disconnected mailbox. These are mailboxe  ...