Migration Manager for Active Directory

Migrating users to Target domain while Exchange mailboxes will remain on Source Domain

I am using MMAD and I will be migrating users to a new domain using Migration Sessions, but the users Exchange mailboxes will remain in the Source Domain.

Do I need to use MMEX if I am not moving the mailboxes? As I will be migrating all users in several migration sessions, do I need to have Directory Sync?

Should the User IDs on Source Domain be disabled after migrating the users to Target Domain?

Thank you,