Office 365 Migration - Azure AD before Mailbox migrations?

Hi Brendan,

1. if you are decide to use a cutover migration, there will be a kind of one-time sync for your on-premises identities to Azure AD. You have to disable directory synchronization in your tenant prior you can start migrate your mailboxes. Important hint: if you need directory synchronization after migration, you cannot decommission your Exchange on-premises server. Some attributes are set to read-only in Exchange Online if directory synchronization is enabled. And the current supported attribute change, i.e. for proxyaddresses, is to use an on-premises Exchange server. This means you cannot get rid of your on-premises infrastructure.

2. AFAIK you cannot migrate to existing users, it will throw an error during the first synchronization. You can merge it with Azure AD Connect but then you cannot use a cutover migration.

Regards,

Dominik