Microsoft 365 and another email provider on the same domain
Microsoft 365 can share your domain with another email provider, so the people who rely on Teams or Exchange keep their Microsoft mailboxes while everyone else moves to a provider that charges only for email. Microsoft calls this an internal relay domain. These are the exact steps, with GINbox as the other provider.
Updated 27 October 2026.
How it works
Microsoft’s documentation on accepted domains explains that with an internal relay domain, recipients can be in Microsoft 365 or on another email system: known recipients get their mail in Microsoft 365, and the rest is relayed through a connector you create. Without that connector, the other recipients do not receive mail.
Before you start
- You are an Exchange administrator for the Microsoft 365 tenant, and the domain’s MX records point to Microsoft 365.
- In the GINbox admin panel, list the people who stay on Microsoft 365 (Mailboxes, “Keep some mailboxes at Google or Microsoft”) and enter your tenant’s
.onmicrosoft.comname. GINbox shows each person’s routing address. - Create the other people’s GINbox mailboxes and bring their mail across: moving from Microsoft 365.
Set it up in the Exchange admin center
- Set the domain to Internal relay. Go to Mail flow → Accepted domains, open your domain, and under “This accepted domain is” choose Internal relay. Save.
- Add a connector to the other provider. Go to Mail flow → Connectors and add a connector from Office 365 to a Partner organization. Use it only for mail sent to your domain, route it through the smart host
mail.ginbox.net, and always use TLS. Microsoft’s guide: set up connectors to route mail. - Remove the moved people’s Microsoft 365 mailboxes once their mail is copied, so Exchange Online no longer recognises them and relays their mail.
- Fix SPF and test. Publish one SPF record naming both providers, for GINbox:
v=spf1 include:spf.protection.outlook.com include:_spf.ginbox.net ~all. Send a test from outside to one person on each side, and one between the sides.
If GINbox receives first instead
- Point the domain’s MX records to GINbox.
- In the Exchange admin center, add a connector from a Partner organization to Office 365, identified by GINbox’s sending addresses (the GINbox admin panel lists them), so Microsoft accepts the mail GINbox passes on for the people who stay.
- List the people who stay in the GINbox admin panel. GINbox filters everyone’s mail first and delivers theirs to Microsoft 365.
Why do it, and what it saves: Keep a few on Microsoft 365, the rest on GINbox.
Questions people ask
Can Microsoft 365 share a domain with another email provider?
Yes. Set the domain to Internal relay in the Exchange admin center and add a connector to the other provider. Exchange Online delivers mail for its own mailboxes and relays mail for everyone else to the other provider.
What is the difference between Authoritative and Internal relay?
Microsoft explains that with Authoritative, mail is delivered only to recipients in Microsoft 365 and unknown recipients are rejected. With Internal relay, recipients can be in Microsoft 365 or elsewhere: known ones get their mail in Microsoft 365, the rest is relayed through a connector.
Do the people on the other provider need a Microsoft 365 licence?
No. They must not have Microsoft 365 mailboxes, so Exchange Online does not recognise them and relays their mail. You keep licences only for the people who stay.
How does mail from the other provider reach Microsoft 365 users?
Every Microsoft 365 mailbox also has an address at your tenant’s .onmicrosoft.com domain. The other provider delivers mail for the people who stay to that address; GINbox does this for each person you list.
Try it on your own domain
Set up a real mailbox at your own domain, with every record checked for you. No card needed.