1. Open the EAC
Click on start and then expand Exchange Server 2016 and click on Exchange Admin Center.
2. Login to the EAC
If you have a certificate warning, click yes to continue and then enter the credentials to login and press enter.
3. Click on Mail flow
On the left hand side, click on Mail flow.
4. Click on Accepted domains tab
On the top right, click on accepted domains.
5. Click the + button
Click the + button to add in a new accepted domain and follow the steps till completion.