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Switch default domain in Office365

Hi,

following situation:

We have an Office365 tenant testTenant.onmicrosoft.com.
There are some users that have licenses for Office Online and Exchange Online.
Those user names are as follows:

user.name@testTenant.onmicrsoft.com

The users habe One Note books and files.

Now we added our custom domain test.de zu the Office 365 tenant and verified it.
We also have a on Premise AD domain "test.de" which is connected to the Office 365 tenant with AAD Connect and Standard-Settings.

What happens now:
- We create a user in on premise Active Directory.
- We set the Attributes "proxyAddresses" and "mailNickname":
   - proxyAddresses: SMTP:user.name@test.de
   - mailNickname: user.name
- Now we sync the AD to Azure AD (AADConnect
   - The user is created in Azure AD as user.name@test.de
- As soon as I add the license for Exchange Online the mailbox gets created.
   - The user then has following e-Mail-Addresses:
     Primary: user.name@test.de
     Additional: user.name@testTEnant.onmicrosoft.com

That`s fine and what I want.
Now I saw, that the configured default Domain ist still "testTenant.onmicrosoft.com".

What exactly does that mean?
We are planning to put the MX record for test.de to Office 365 in the future. Would it be a problem that the default domain is still testTenant.onmicrosoft.com?
What would happen, if I change the default domain to "test.de"?
     - Will the old users (user.name@testTenant.onmicrsofot.com) stay untouched? (user-Login-Name, E-Mail-Addresses, user data)
     - Which E-Mail-Addresses would new users get, when I enable the Exchange Online license? (Only @test.de or also   testTenant@onmicrosoft.com too)

What would you prefer? Switching the default domain?

The goal is to delete all the old testTenant.onmicrosoft.com Accounts after our migration is done.

Man thanks in advance!
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Systemadministration
Asked:
Systemadministration
1 Solution
 
Vasil Michev (MVP)Commented:
You can safely change the default domain, old users will not be impacted. New users will get both domains added as aliases.
1
 
Todd NelsonSystems EngineerCommented:
Why do you want to remove the *.onmicrosoft.com domain?

Review the "Renaming or removing the onmicrosoft.com domain" section in this article ... https://support.office.com/en-us/article/About-your-initial-onmicrosoft-com-domain-in-Office-365-b9fc3018-8844-43f3-8db1-1b3a8e9cfd5a?ui=en-US&rs=en-US&ad=US

If you can't remove the *.onmicrosoft.com domain there must be a reason behind why each user has a *.onmicrosoft.com email address associated with their account.  Do a bit more research to find out what the ramifications might be if you do remove it.  Contact support and ask them too.
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SystemadministrationAuthor Commented:
Hi,
I was able to change the default domain without problems.
I only had the re-configure the  AAd Connect tool.

Thanks!
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