By default Microsoft Exchange Server 2016 configures a default accepted domain and email address policy using the fully qualified domain name (FQDN) of the Microsoft Active Directory (AD) domain into which it is installed. If necessary, you may use the following commands in the Microsoft Exchange Management Shell (EMS) to create an SMTP domain for Microsoft Exchange Server 2016. Note that you may change the -MakeDefault value to $false if you do not want it to be used as the default SMTP domain for the email address.
New-AcceptedDomain -Name domain.com
-DomainType authoritative |
Set-AcceptedDomain -MakeDefault $true