Exchange On-prem migration to Exchange Online

Damian M 0 Reputation points

I'm looking for best practices and or advice for migration of our On Prem Exchange 2016 to Exchange Online with Full-Cut-over migration.

We currently utilize Azure Ad Connect on prem for our computer and accounts to sync to Azure AD.

I've read that there may be issues or gotchas to look out for when migrating, but looking for further advice and or best practices or from the field experiences.

I've read that if using Azure Ad Connect, I'll need to prepare our environment by either setting the msExchMailboxGuid attribute to NULL in the Azure AD Connect setting.

  1. Disable Azure Ad Connect Sync
  2. Set Attribute to Null
  3. Enable Azure Ad Connect to Resync

By doing so, allows us to pre-migrate our on prem exchange mailbox data to exchange online, granted that licenses are assigned. Once we are ready to migrate we just need to make changes to our MX records / DNS records to point to Exchange Online.

Is this valid and has anyone tried this method?

Does Azure AD Connect still continue to work? If not, what is the alternative method? As we still maintain on prem AD environment, I assume we will still need this functionality.

Any advice/reference documentation would be helpful.


Microsoft Exchange Online
Microsoft Entra ID
Microsoft Entra ID
A Microsoft Entra identity service that provides identity management and access control capabilities. Replaces Azure Active Directory.
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  1. Kael Yao-MSFT 37,586 Reputation points Microsoft Vendor

    Hi @Damian M

    Cutover Migrations are not allowed if dirsync has been enabled for the tenant.

    While you can re-enable dirsync once the cutover migration is done.

    There have been several threads discussing this topic, which may be helpful.

    For your reference:

    Disable DirSync - Perform cutover migration - Re-enable DirSync

    Exchange cutover migration will work with AAD connect sync user?

    Using Dir Sync after Cut over migration

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