Log into your Windows server via RDP.
Start -> Administrative Tools -> Configure Your Server Wizard -> Next -> Next
Click on Mail server from the list and then Next. There are two types of Authentication methods, the Local Windows Accounts and Encrypted Password File. With the first one, you can not have two same users by all email@example.com@seconddomain.com
That is not possible if you choose Local Windows Accounts, because you can not have two "admin" accounts. With Encrypted Password File it's possible to do the above. We will be using the second method for the built-in Mail Server installation.
Enter a domain name and click Next.
Click Next once again, supply the I386 destination and let the installation run.