vpopmail is Inter7.com's POP mail server for Qmail and Postfix. vpopmail uses the UNIX maildir format for storing messages, so it is compatible with any programs that use this, including mutt and Courier IMAP. Because vpopmail works well with Courier IMAP you can use the two as a mail solution for a small to medium enterprise.
vpopmail scales to 23 million virtual domains, supporting 23 million mailboxes on each. It also supports multiple database backends for storing authentication information and aliases. However, it provides no out of the box clustering functionality, and the maildir storage sytem would almost certainly push the host operating system to its limits with such a large number of users.
vpopmail does not resolve names through the Name Switch Service (traditionally done through /etc/passwd). This also means it does not support NIS, Active Directory or any of the other cool NSS functionality available to a modern Linux admin.
If you're looking for a fast, scalable POP server, vpopmail may be for you. vpopmail is supported on Linux and FreeBSD and compiles on many other UNIX variants. It does lack the functionality of Cyrus and I find its configuration files kludgy, although many admins find it comfortable and easy to configure.