I keep going around in circles when trying to plan a migration from Exchange 2010 to Office 365 with conflicting information and different opinions to which is the best migration method for my situation.

Consider this scenario and please advise....

A company has an AD domain and domain name ABC.com,

Within ABC.com, there is a single exchange 2010 server that hosts all the exchange roles for 300 users,

ABC.com also own and use several email domains such as DEF.com, LMN,com and XYZ.com (which all are setup as accepted domains and MX/AutoDiscover records point to the same on-premise server).

Each user has an ABC.com alias with another accepted domain set as their primary. Each users logs into Outlook and other domain authenticated services with their @ABC.com UPN.

30 mailbox users that use the XYZ.com domain as their primary alias want to move to office 365, followed by users of the DEF.com a few weeks later, followed by LMN.com a few weeks after that.

Finally ABC.com will be required to route via the 365 environment where it should be set as an alias of the mailboxes (as it was on the on-premise server originally)

ADFS will need to be implemented to allow for account management via the local AD

What would be the best way to achieve this goal? Best Migration Path? Steps?

Would ABC.com need to be registered as an accepted domain in office 365 to have ADFS and DirSync to work? - (I can't see it working without this but i need to ask)

As the ABC.com DNS records cannot be changed until the very end of the migration, I assume, whilst migrating the accepted domains, in order to receive emails on both ABC and XYZ domains, both alias's will need to be configured in Outlook separately (i.e. [email protected] pointing to office 365 and Use[email protected] pointing to the on-premise server) - is this correct?

2 Answers 2


Please have a look on below given informative resources that might helps you to get in more details and lets you to get this job done:

Support for Multiple Top Level Domains:- https://community.office365.com/en-us/w/sso/support-for-multiple-top-level-domains

Setting up AD FS and Enabling Single Sign-On to Office 365:- blogs.technet.com/b/canitpro/archive/2015/09/11/step-by-step-setting-up-ad-fs-and-enabling-single-sign-on-to-office-365.aspx

Directory synchronization roadmap:- technet.microsoft.com/library/hh967642.aspx

If you want to deploy hybrid, you need to deploy DirSync. Please refer to Exchange Server Deployment Assistant for further information according to your requirements it will create deployment instructions:- technet.microsoft.com/en-us/exdeploy2013/Checklist?state=2718-W-AAAAAAAAQAAAAAEAAAAAAAAAAAAA&startOver=True

Office 365: ADFS - Support for Multiple UPNs:- www.msexchange.org/blogs/walther/news/office-365-adfs-support-for-mutiple-upns-724.html

Hope this helps!

  • I'm removing the reference to the product.
    – Vick Vega
    Oct 28, 2015 at 3:51
  • To summarize: You would have to validate all the smtp domains in Office365, utilize ADConnect to populate the Tenant with the user objects and utilize ADFS. The important part is multi-domain settings in ADFS. The next step would be to switch domains one by one to Office365 while making sure that primary SMTP address is the one that defined as primary from on-prem Exchange. blogs.technet.com/b/askpfeplat/archive/2013/09/02/… My recommendation would be to utilize Exchange 2013 as Hybrid server.
    – Vick Vega
    Oct 28, 2015 at 4:06
  1. The best way to move the users in batches is to set up a hybrid environment between your on-premises organization and exchange online.
  2. All domains need to be added and verified in Office 365 before migrating.
  3. While the migration is still in process (you haven't moved all mailboxes to Office 365) autodiscover needs to point to on-premises
  4. Keep MX records pointing to on-prem. When a user is migrated it is automatically stamped with a target address ([email protected]) based on which mails will be routed to their cloud mailbox.

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