(This guide was contributed by Hans Sandsdalen, the operator of a site that uses qmail with Citadel.)
Install qmail. See Life with qmail, Installasjon av qmail, or any of the other qmail installation howtos available on the web.
In this configuration, Citadel's citmail utility lets qmail feed messages into the Citadel server. For a user that wants this, we put it into his .qmail file.
$HOME/.qmail (username must be changed): | /usr/local/citadel/citmail username
Note: This approach just works with local unix users. Qmail is rather hard to set up, and due to its license most distributions don't ship packages. If you're not yet experienced with qmail, you might have a look at qpsmtp or Postfix.