Removing the default domain in Exchange Server 2016 using Exchange admin center.
When Exchange Server is deployed the default domain is going to be the FQDN of the Active Directory domain, however some customers have a non-routable domain (for example patricio.local) and this type of domain should be removed as soon as the production domain(s) are added to the Exchange Organization.
If the administrator tries to remove a default domain the delete button will not be available. In order to get there, click on mail flow, accepted domains and select the domain from the list.
Before removing the accepted domain, we must make sure that domain is not being used in the e-mail address policies. In order to check go to email address policies and double click each entry from the list.
The goal of this task is to make sure that the domain that we want to remove is not listed on the email address format, as shown in the image below.
After performing all those tests, we can go a domain that will stay in the Exchange Organization and select Make this the default domain (we are back at the Accepted Domains tab) as shown in the image below.\
Finally, we can select the former default domain and the delete button will be available, click on it.
A dialog box asking for confirmation will be displayed, click on yes to confirm.
If the domain that is being deleted has been in production for a long time, including the email address polciies, then it is most likely that your users have the default domain stamped on the users’ mailboxes. In that case it is better to remove such entries from the mailboxes, you can do it manually, PowerShell or ADModify utility.