What is an Alternative Suffix? By default, When you create a 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 this case, ABC.com acquires a company called XYZ and they are willing to give separate UPN for the XYZ people as Mail Domain(XYZ.ABC.com) without investing in any new setup or Active Directory setup which means the company wants to have separate Subdomains for the Different Business processes.
It is truly possible using Alternative UP Suffix which will add an 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 the Exchange Control Panel(ECP) and validate whether you’re able to see the new Alternative UPN to setup as User Logon name.
After all is set in the active directory and Exchange Side, you further have to set up MX record and modify Exchange gateways to receive emails for the new subdomains or new suffixes.
The most important thing is you need to add Azure Active Directory for the Authentication to work for the new Subdomain, Exchange Online side as the Accepted domain, and Enable AADConnect to sync the new SMTP addresses.