Do you mean you want to add a new domain to Exchange Server? For example, add the xxx.domain2.com email address to your Exchange Server(domain1.com)
Based on my knowledge, you could add your domain to Microsoft 365 or Office 365 called accepted domain his functionality of an accepted domain means that users in this domain can send and receive mail. For more information on how to add your domain to Microsoft 365 or Office 365 using the Microsoft 365 admin center, see Add a domain to Microsoft 365 or Office 365.
After you add your domain using the Microsoft 365 admin center, you can use the Exchange admin center (EAC) to view your accepted domains and configure the domain type. For more detailed about its steps, you could read it.
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