Here is information about how to add new user who has a different domain name in their email address.  For example, if you use @abc.com and the new user has an email ending in @def.com:

1 – Click on the Users tab

2 – Click on Add New User

3 – Enter the person’s full name

4 – Scroll down a bit then click on Add New Domain

5 – Type in the new domain name (using the example above you would type def.com)

6 – Click on the Add button

7 – Type in the first part of the email address

8 – Use the dropdown menu to choose the new domain that you just added

9 – Click Next then finish choosing other services for the user and choosing the type of account (User, Group Admin or Administrator) then hit Add