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. User1@XYZ.com pointing to office 365 and User1@ABC.com pointing to the on-premise server) - is this correct?