How to have a 3rd party host an Exchange email of the same name as an Azure AAD custom domain

Michael 76 Reputation points
2023-04-24T14:15:03.77+00:00

I have an Azure account with a tenant that has a custom domain (hosted by a different domain provider). I want to have another 3rd party to host a Microsoft Exchange presence that has the same name as the custom domain on Azure. However, the 3rd party Exchange provider says I can't do this because it would create a second Active Directory of the same domain. So, how can I have an Azure presence (with a web server) and a 3rd party Exchange server use the same domain? Or, that's not the way it works?

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Microsoft Entra ID
A Microsoft Entra identity service that provides identity management and access control capabilities. Replaces Azure Active Directory.
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Accepted answer
  1. RevelinoB 2,775 Reputation points
    2023-04-24T14:30:57.09+00:00

    Hi Michael, In this scenario, you would need to set up a split-brain DNS configuration. This allows you to use the same domain name for both your Azure web server and your 3rd party Exchange server, while still maintaining separate Active Directory instances. Here are the general steps to set up a split-brain DNS configuration:

    1. Create a new subdomain for your Azure web server, such as "web.yourdomain.com".
    2. Create a new DNS zone in Azure for the "web.yourdomain.com" subdomain.
    3. Add a DNS record to the "web.yourdomain.com" zone for your Azure web server's IP address.
    4. Configure your Azure web server to use the "web.yourdomain.com" subdomain for its web traffic.
    5. Set up a split-brain DNS configuration with your domain provider, so that requests for "yourdomain.com" are resolved to the Azure web server's IP address when coming from the public internet, but are resolved to the 3rd party Exchange server's IP address when coming from within your network. By setting up a split-brain DNS configuration, you can use the same domain name for both your Azure web server and your 3rd party Exchange server, while still maintaining separate Active Directory instances.

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