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Internal Relay mailflow setup in Exchange 2016

Let’s take a look at a scenario where Orgn A ( acquired Orgn B ( and you are tasked with setting up Orgn A’s exchange server to accept all emails from the internet for both the Org’s

This also holds true when you want to migrate all users to a new new Exchange server users with a different SMTP address than the old exchange. Users will continue with their old SMTP address until they migrate the mailboxes over to the new Exchange 2016 server.

New Exchange 2016 – domain: Exchange 2016 server in domain Exchange 2013 is already in production with the domain

Old Exchange Server 2013 – domain: Exchange 2013 is already in production with the domain

First step is to create Accepted domain in the New Exchange 2016 Server. Here we have to create the old domain “” as an accepted domain in the New Exchange 2016 Sever.

Open Exchange Admin Control by navigating to: https://localhost/ecp . Then navigate => Mail Flow => Accepted Domains=> Click on New +

Then new Accepted domain window, provide the name for Accepted domain ( and select the domain type as “Internal Relay” and click Save.

Internal Relay: In an internal relay accepted domain, Exchange server of the company hosts some mailboxes and rest are hosted elsewhere. This is useful when there is a shared SMTP domain between two organizations where few mailboxes exist in one company and rest on other server. When internal relay is configured, exchange server will try to find mailbox in existing Exchange server first and if it doesn’t find it will forward to the smart host specified in the send connectors.

Now that we have the Internal Relay in place we need a send connector to be able to route the mails to the Exchange 2013 OLD server. This route is very important as this is the path to route emails to the Old exchange server.

In the EAC navigate to Mail flow => Send Connectors => click new + . Then type the name “route to Ex2013” => leave the option as internet => Click Next => Under network settings select “Route through smart hosts” => click the + sign and Add => Add the IP address of the Exchange 2013 Server => Click Next => Add address space click the + sign => Add the domain “” in address space +. Add the server => click Next and finish.

In summary, now when we send an email to the user domain the Exchange 2016 will look up for the user in the local Exchange server. If the user exists it delivers the mail if not it will route the mails to the old Exchange 2013/2016 server as per the above configuration.

Ratish Nair

Microsoft MVP | Office Servers and services


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