Adding a custom domain - Will it cause M365 email to route internally while my MX is pointing to Gsuite?

Question

I have a customer with mailboxes sitting on gSuite, MX recording pointing to gsuite
I want to migrate them to M365 - I have created an onmicrosoft.com tenant for them already in Office365.
I want to add a custom domain to the M365 portal, and verify the domain. By doing so.... will it cause emails to stop working?
MY understanding is... other M365 tenants will see my custom domain, and try to route directly to my exchange online, rather than checking the MX record and sending to gSuite.
Will adding a custom domain to my M365 cause emails routed through M365 to stop being delivered?

Answer 1

Hi @Paul Dacre ,
You could follow the steps below to migrate:
Preparation Prerequisites: Google Workspace migration prerequisites in Exchange Online

1. Create a subdomain for mail routing to Microsoft 365 or Office 365
2. Create a subdomain for mail routing to your Google Workspace domain
3. Provision users in Microsoft 365 or Office 365

Migration steps: Migrate consumer Google Workspace (formerly G Suite) mailboxes to Microsoft 365 or Office 365

1. Verify you own your domain
2. Add users to Microsoft 365 or Office 365
3. Create a list of Gmail mailboxes to migrate
4. Connect Microsoft 365 or Office 365 to Gmail
5. Create a migration batch and start migrating Gmail mailboxes in Classic Exchange admin center (Classic EAC)
6. Update your DNS records to route Gmail directly to Microsoft 365 or Office 365
7. Stop synchronization with Gmail

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Answer 2

Hi @Paul Dacre ,

Sorry for the delay in answering your question, emails in O365 will only check MX records and are not routed directly internally, so adding a domain to O365 will not cause emails to stop working.

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Answer 3

In my experience as soon as you add a custom domain to 365, Microsoft will start routing some MICROSOFT domain email such as support request responses internally to your tenant, and there is no way to then route them from your tenant to on-prem. You may want to use your onmicrosoft.com account or another external email domain account if you need to contact support in the interim before you start using the custom domain.

Also if you have another custom domain already in use in Microsoft 365 in that same tenant, you will need to change 365 to relay for your unconfigured local domain. In the 365 Exchange Admin > Mail flow > Accepted domains > select your in-use hosted domain and set it to Internal Relay.

As stated by Jame Xu-MSFT, other Microsoft tenants will honor your published DNS records and route correctly. It will mainly be an issue among some of Microsoft's servers - not all of them - for example licensing emails will get to you, but not support requests.

It may also become an issue setting up a new remote user in Outlook since AutoDiscover will check for a 365 tenant first. The workaround is to set HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover\ExcludeExplicitO365Endpoint to 1. Just make a note to remove that once you do start using the local email domain in 365.