Types of Domain
There are three types of domain: standard, alias and backup. A standard domain is the only type of domain which has users and mailing lists. An alias domain allows redirection of all mails for that domain to either another local domain or local email account. A backup domain is usually only used for a mail server acting as a backup of another mail server.
Usually you may only require the use of a single domain and this is usually a standard domain. However, if you plan to host multiple domains and require more complex mail routing, you can mix the various types of domain in order to achieve the desired effect. Here is a selection of examples of how best to use domains.
Single Domain Hosting
If you have a single domain and only wish to host some users and possibly a few mailing lists, you only need to create a single standard domain. This will allow you to add users which provide email accounts on that domain.
If you own multiple domains and wish to combine some of the domains into one (i.e. firstname.lastname@example.org, email@example.com...etc should deliver to the same account). Then you can create a single standard domain (to host your accounts) and then create as many aliases for that domain as required. Each user on the standard domain will then effectively have multiple alias email addresses assigned (one for each alias domain) and will be able to accept mail as firstname.lastname@example.org, email@example.com, etc.
If you want this mail server to act as a backup SMTP service for another mail server, creating a backup domain and static route will allow this. A backup domain signals to the SMTP to accept any email addressed to that domain so it can be passed onto the Outgoing Mail service. Also, it is perfectly fine to have an alias domain for a backup domain.