O365 tenant migration


Hey Everyone,


I am trying to come up with the right plan to do a full O365 Tenant migration in our organization.

So far I was exploring a lot of articles etc. how to plan this quiet a large task in our organization. Currently we are limited on resources is well etc. so all the work needs to be done plainly by us.


Today I was reading this article which clearly explain the steps that you should take to migrate all users in one go to the new tenant however it looks like there is a downtime in this process. Obviously I do not expect that this type of task can be done without any downtime but lets come down to my questions.


First I will describe quickly our environment. We have a US site and also EU site. We also have Skype For Business on-prem and there is a one-way trust setup between our domains. Do not ask me why it was set up this way - it has been done before I have join the team. On our current tenant we have two domains setup - one is our main domain and the other is the new domain that we will use across the company. For the purpose of description lets call:


- contosoA.com - US site

- contosoB.com - EU site


These two domains are setup in our current tenant in EU - lets call it:


- tenantA


We have about 1000 users in EU and about 300 in US. We are running Exchange-Hybrid environment is well. About 400 users are migrated to O365 in EU.


My problem is that because two domains are taken already by "tenantA" I cannot move the main domain to the new tenant which is "tenantB" and domain "contosoB.com".

I was thinking if we can use some sort of SMTP relay function so we will be able to migrate user to the "tenantB", however leaving the smtp setup on "tenantA" and saying that:


user@contosoA.com any email will be forwarded to user@contosoB.onmicrosoft.com


After we will migrate all the users etc. I will reset all the user records on old "current" tenant to onmicrosoft.com and then I will be able to verify the domain on the new tenant.


Is this a correct approach, is it possible to setup this type of mail routing between two tenants?




4 Replies

There's nothing that Microsoft offers in that space, you'll have to look into third-party products.

Hey@Vasil Michev ,


Would you have any recommendations. We were thinking to setup third-party SMTP Relay. Would you have any other recommendations?




We try to keep these forums advertising-free, but there are few vendors out there that do tenant-to-tenant migration staff and/or have products that handle mail flow in such scenarios - look them up.

@mrktos, I have been in your situation several times, dealing with mergers for my clients and needing to move them between tenants.  The only option has been third-party products that perform the migration for you.