This article applies to Web Hosting and Custom Email accounts only.
The secondary domain names are added to your mail server as aliases to your primary domain name. This means that email coming to your secondary address will be delivered to the mailbox on the mail server for your primary domain.
Let's say you have a primary domain name ABC.COM with the full-blown email server, and you also have a second domain name (XYZ.COM) registered for the same IP address.
Now, you want people to be able to send messages to the user John at XYZ.COM (email@example.com). To make this possible, you need to create a new user John at abc.com (firstname.lastname@example.org). After you have done this, all mail to user email@example.com will be delivered to the mailbox firstname.lastname@example.org.
Please remember that additional domain names are aliases for your main domain, and the mail server treats them as such. In other words, mailbox email@example.com is an alias for the mailbox firstname.lastname@example.org, meaning it is the same mailbox.