Multi-Geo & Multi-Domain Setup

JillArmour / rohabbrar ,

I'll see if I can throw in any useful info too.

As blakecheek mentioned (and as I assume is implied by the end of the question) definitely seek some MS advisory hours / support. I don't think just posting here will result in a call, contact your Technical Account Manager directly.

For the technical aspects, such as migrating all the mailboxes: we're in the process of doing that ourselves (mainly between Canada and several distinct US sites). I agree, use a tool like https://www.bittitan.com/migrationwiz/why-migrationwiz/ to combine the tenants. It can move the ExO mailboxes and SharePoint/OneDrive data separately. There's a https://help.bittitan.com/hc/en-us/articles/115008112447-DeploymentPro-Guide package they have which can take care of switching over MS Outlook from the old to the new tenant.

There used to be a delay in migrating the actual domain from one O365 tenant to another, but I think that has been shortened now. Regardless there will probably be some small outage since (unless O365 has relaxed the rules) you can't have a single external domain associated with multiple O365 tenants. Of course users/mailboxes within the same (combined) tenant can have different email domains (as primary or secondary/proxyAddresses). So all the combined users can be @xxxx.net  and also retain @xxxxx.ie or whatever.

Migrating Azure resources will depend on the current config: there's data movement to move data between regions, so a cost involved. I'd start by contacting MS to have the Subscriptions moved to the same account/tenant, and then you can combine where it makes sense. Moving https://learn.microsoft.com/en-us/azure/azure-resource-manager/management/move-resource-group-and-subscription is easy for the most part if you want to consolidate. But need to have the Subscriptions moved to the same tenant first.

NOTE: as this IS the MS Partner Forum, I should add that you WILL need to maintain separate O365 tenants for CSP/Partner Center access in each of the regions MS does business. I don't know how countries are divided up in Europe, but in North America we'll need to retain separate Canadian and US tenants as the clients for each country are managed from their respective country's portals. So while your US HQ can manage all your US customers, after you have moved all your xxxxx.ie and xxxxx.co.uk domains to the US HQ tenant for the purposes of emails, etc. You'll still need to maintain your xxxxxxxie.onmicrosoft.com domain in Ireland for Irish customers and (probably) your xxxxxxxuk.onmicrosoft.com domain for your UK customers (and each other country, unless MS EU rules are different?). Only users who need to manage these customers need to retain some username @ whatever.onmicrosoft.com logins to each CSP/Partner Center.

I hope that helps,

    --Saul