Adding Domain to Existing O365

New Contributor

My company recently acquired another company with about 60 users. We use Exchange Online for our email with an on premise AD and run a DirSync setup.


The smaller company currently uses O365 for much more than we do, including their email. Since we both have Microsoft hosted email, how would I go about absorbing their email and adding their domain?


I know enough to be dangerous, but this is something new for me. Do I need to do something with MX records from their domain host as well? Maybe this is too complex, but I figured try here and see what info I can get on merging our email into one entity.

4 Replies
best response confirmed by dalbright (New Contributor)

Hi @dalbright 


As far as I'm aware, there isn't a easy native way to join to different tenants together, though as more businesses sign up to EOL and this becomes more common, I'd hope that MS introduces a mechanism to do this (as well as splitting up a tenant).


The only thing I have in my notes is a document from on how to do this manually


I've not done this myself - we were looking into it as a client had the same requirements as you, though in the end they decided to keep the tenant separate.

Agree with the above, there's nothing useful that Microsoft currently offers in that area. There are however some third-party products that can automate the whole process, of course at a price. Look them up if you are interested.

Thanks. What I'll do is consult my vendor and see if they have any recommendations as I'm sure they've had to deal with this many times before, just wasn't sure if it was something quick and easy.

Easy.. add their domain as an accepted domain in Office 365
You will need to get a export of their AD user list in a csv file.
Create AD users in your AD.
Export their .PST file and uploadt it to the azure blob spot and use the pst import mapping file to map those pst files to their new mailboxes.