On Demand Migration (ODM) has been designed as a complete and easy to use Office 365 tenant to Office 365 tenant solution. ODM is able to use the information obtained through discovering, migrating and/or mapping user objects to automatically handle mail forwarding/routing through the different stages of the migration process. Here is a brief overview of how ODM handles mail routing/forwarding.
Initial mailbox migration
During the first mailbox migration of a user, or collection of users, ODM will set forwarding on the new target mailbox.
Target mailbox -> Source mailbox
ODM assumes that end-users will not be using their new target mailboxes immediately after the first "Migrate Mail" task (cut-over), so forwarding is set on the new target mailbox to forward any new mail that is received in the target back to the source. This way any new mail is assured to be delivered to the active end-user mailbox. ODM will only automatically set forwarding during the initial Migrate Mail task, any future change to mail forwarding will require running the "Switch Mailboxes" task.
However, should your particular migration project call for end-users to begin using their new target mailbox immediately, you can run the "Switch Mailboxes" task right after the initial "Migrate Mail" in order to tell ODM to switch the forwarding from Source mailbox -> Target mailbox.
Migration pass 2
During this optional step, additional or newer mail is migrated from the source to the target mailbox. Usually, at this point the end-user is still actively using their source mailbox. Mail forwarding is still set from Target mailbox -> Source mailbox, as was automatically set during the initial mailbox migration.
Usually, at a predetermined point in the migration process, the end-users are "cut-over", or switched to actively use their new target mailboxes. This is traditionally where the "Switch Mailboxes" task comes in to play. The "Switch Mailboxes" task will set forwarding from Source mailbox -> Target mailbox, so that new mail will now arrive in the target mailbox. Additionally, the "Switch Mailboxes" task will activate the Client Update Agent to seamlessly switch their Outlook profile on their local workstation to their new target mailbox.
How do I know where mail is being forwarded?
On the ODM Mailboxes screen, there is a "Switched" column, which will state where new mail will be delivered (forwarded):