What is Alternative Suffix?
By Default, When you create user in Active Directory, Users will get the Root domain UPN as Login ID. Consider you have a root domain named ABC.com, users will get User@ABC.com as login and UPN Suffix.

In case, ABC.com acquires company called XYZ and they willing to give separate UPN for the XYZ people as Mail Domain(XYZ.ABC.com) without investing any new setup or Active Directory setup which means the company want to have separate Sub domains for the Different Business processes.

It is truly possible using Alternative UP Suffix which will add Alternative login UPN Suffix for the user accounts. Let’s follow the below mentioned way to get it done..

Open Server Manager –> Open Active Directory Domains and Trusts

Right Click on Active Directory Domains and Trusts — Click on Properties

Enter the Alternative UPN Suffixes and click on add and apply

Up on the completion of the Additional UPN Suffixes adding in the Active Directory side, Login to Exchange Control Panel(ECP) and validate whether you’re able to see the new Alternative UPN to setup as User Logon name.

After all set in the active directory and Exchane Side, you further have to setup MX record and modify Exchange gateways to receive mails for the new sub domains or new suffixes.

Most important thing is you need add in Azure Active Directory for the Authentication to work for the new Sub domain, Exchange Online side as Accepted domain and Enable in AADConnect to sync the new SMTP addresses.