Resolution
Will Exchange Migrator update a source Distribution List with the Custom Recipient that it creates during the mailbox migration?
fact
Exchange Migrator 2.x
fix
As of Exchange Migrator 2.3, EM will now update the source Distribution List with the Custom Recipient created during the mailbox migration.
Prior to 2.3, the hidden mailbox remained a member of the source Distribution List. However, note that mailflow is always maintained because of the following SMTP routing. If you email the source Distribution List (DL), then the email goes to all of its members. If a member has been migrated, then the hidden mailbox is still a member of the source DL. After the mailbox migration the hidden mailbox has an alternate recipient which points to the source Custom Recipient (CR) that Exchange Migrator (EM) created in the source during the mailbox migration. The CR then forwards the emails to the forwarding address, which in this case will be the target desired mailbox.