By default, when you deploy a new Exchange 2016 organization in an Active Directory forest, Exchange uses the domain name of the Active Directory domain where Setup /PrepareAD was run. If you want recipients to receive and send messages to and from another domain, you must add the domain as an accepted domain. This domain is also added as the primary SMTP address on the default email address policy in the next step.
Enter your user name and password in Domain\user name and Password and then click Sign in.
Go to Mail flow > Email address policies. On the Email address policies page, select Default Policy and then click Edit
On the Default Policy Email Address Policy page, click Email Address Format.
Under Email address format, click the SMTP address you want to change and then click Edit
On the Email address format page in the Email address parameters field, specify the SMTP recipient domain you want to apply to all recipients in the Exchange organization. This domain must match the accepted domain you added in the previous step. For example, @contoso.com. Click Save.
In the Default Policy details pane, click Apply.
For Azure VM hosted Exchange and hardened VM (Prerequisite):
Change the instance IP
Open Port 25 and 443 in Network Security Group (Classic)