When you select the "Domain names and Internet mail configuration" option on your system administration screen, you can configure several different types of domains. These are explained here.
Local host aliases are all the names by which your host is known. One server may serve several domain names (like a mycitadel.com and mycitadel.org); enter yours here. Don't forget to have a look at your DNS configuration.
There are several reasons why you might not want your Citadel to deliver outbound mail directly to the Internet. For example, you might be using a provider that blocks port 25, or you might be sending mail through a security service that scans and filters messages. Or perhaps there is another MTA on your host or network such as Postfix and you want it to handle mail delivery. For any of these reasons, you can configure a Smart Host through which all outbound email will be sent.
You can just specify a host name, or an IP address, in which case mail will simply be sent to that host. Or you can specify any valid SMTP or SMTPS URL, which will allow you to use SSL/TLS, change the port number, or add username/password authentication. For example:
smtp[s]://[username:password]@hostname.domain.com[:portnumber]
If you enter multiple Smart Hosts, outgoing mail will be load balanced between them.
You can check inbound mail against any Realtime Blackhole List (RBL). These services identify sources from which all mail is likely to be spam or otherwise unwanted mail. Citadel can handle these directly without an external spam filter when you enter them here. The FAQ has some examples.
An IP or domain name of a machine you've got the SpamAssassin scanner running, to decide wether you want to receive the mail or reject it at the gates, because it only will try sell you cheap watches.
These are domains from which you want your users to be able to "forge" mail, even though your Citadel host is not the place where email for these domains is normally received. This is an edge case and you normally will not use this feature. If you do, enter those domains here.