In most installations, Citadel does not use your host system's user database (such as ”/etc/passwd”). It functions as a “black box” separate from your operating system, keeping its own user database. This is almost always the desired configuration, because you typically do not want email users logging on to your host system.
However, Citadel does support a special authentication mode in which logins are authenticated against your host system's user database using the PAM Api. We call this “host auth” mode (formerly known as “autologin” mode). This used to be a compile-time option, but now it is a setup time option. You can read the manpage for details, or this chkpw use demonstration
This is especially important if you are upgrading an existing system that authenticates users in this manner: when setup asks you if you want host auth mode, you must answer Yes. Otherwise your users will not be able to log in!
One additional note: “mixed mode” authentication, which refers to support for authentication of users either from the host system or from Citadel's user database on the same system, is no longer supported.