This document explains the (.A) (S)ystem (I)nternet command. 

Lobby> .Aide System Configuration Internet

###                    Host or domain                     Record type
--- -------------------------------------------------- --------------------
  1
(A)dd (D)elete (S)ave (Q)uit  ->

This is a 'clean' set up. Pixel uses the mx method because we have local
users who get e-mail on the system. Basically to get all set up for mail
you simply have to type <a>dd: 

(A)dd (D)elete (S)ave (Q)uit  -> Add

Enter host name: pixel.citadel.org
 (1) localhost       (Alias for this computer)
 (2) gateway domain  (Domain for all Citadel systems)
 (3) smart-host      (Forward all outbound mail to this host)

Which one [1]:

Basically what you're doing here is telling Citadel what any aliases for
your machine are. If your machine were pixel.somedomain.com and you also
had a DNS entry set up byFreakdog (freakdog @dogpound2), you might want to
put in pixel.citadel.org as your alias, so that e-mail sent to that
address won't bounce. 

Gateway Domain:
citadel.org (that's the current domain for Citadel/UX systems)

smart-host:
A host to which you send all e-mail to be forwarded to the Internet. In
most cases you won't need this, however I speak from experience when I say
that if you don't have a static IP and domain name (the citadel.org 
addresses are dynamically assigned) a lot of MTA filters will block
mail from your address. So unless you have a static domain, you'll 
probably want to send mail through a smart host. Usually your ISP will
provide you with that information.

All you have to do is save and you're done.