Located at the top of the User page of the Web-based administration is the Show Domain list box.
The default domain listed is the Primary domain. The Primary domain is directly associated with the node name assigned by your ISP. Each mail account created in the Primary domain will have the node name as part of its full address. For example, if your ISP-assigned domain is company.com, and you create a User Account named "john" in the Primary domain, then the email address for this account would be john@company.com.
Also in this drop down menu you will see any Secondary Domains you may have setup within SLmail. They will be viewed as:
In this case, a company named My Company has the assigned domain name of mycompany.com. They may wish to share company.com's mail server rather than have their own. The company.com administrator could create a secondary domain with the domain name mycompany.com, which would function as My Company's mail server. An account within that Secondary Domain would have the address of jim@mycompany.com.