Legacy messaging systems on Unix have typically relied on a loosely coupled collection of programs -- such as Sendmail, UW or Cyrus IMAP, etc. to deliver and read messages. Each of these programs 'forks' (creates another process) when another user signs on. They also rely on legacy mailbox formats which are not very efficient. Citadel uses a single multithreaded server that can handle a large number of simultaneous connections using a proven 'worker thread' model. It handles all protocols in the same server. And it stores messages in a real database for quick access and single-instance storage. Single-instance store is important because if a message is delivered to, say, 100 different users on your system, it only stores the message once, whereas a legacy Unix mail system would have to store 100 copies.
Furthermore, Citadel is scalable. If you want to build a distributed deployment, for example, you can run WebCit on a separate host from the Citadel server, or even run several of them behind a load balancer.