A new functionality has been delivered as a part of 20161121EX update, you can configure migration project to support scenarios where Active Directory syncrhonization is performed using third-party tools instead of Migration Manager for Active Directory. It can be enabled using Set-DMMExProjectOptions cmdlet.
When this parameter is enabled following changes in behaviour are applied:
1) MAGE doesn't rely on ADLDS database any more and uses only object attributes to identify matching
2) AD part of Mailbox switch which used to be performed by MbRedir handler should be done manually using third-party tools or scripts and prior to mailbox switch via console or MAGE.
3) Mage will use only Powershell connections to interact with target organisation.
Before implementing the following preparations have to be undertaken:
1) Target user has to be pre-created and mailbox-enabled.
2) Source and target objects should have proxyaddresses attribute populated with X500:<TargetLEDN> for source object and X500:<SourceLEDN> for target in order to indicate correct matching.
3) Mail flow redirection should be configured to redirect all mail from target mailbox to the source - TargetAddress on target object should contain corresponding source smtp address smtp:<SourceObject>@<Sourcedomain>. ProxyAddresses on both source and target object should also contain this value.
4) Service account for accessing target Exchange organisation should have following roles:
Prior to mailbox switch you should manually change AD attribute to redirect mail flow. This includes:
1) Populate TargetAddress on source object with corresponding target object smtp address smtp:<targetObject>@<targetdomain>.
2) Cleanup TargetAddress value on target object.
3) Add value from step 1 (smtp:<targetObject>@<targetdomain>) to both source and target ProxyAddresses.