An e-mail client made by Cyrusoft, the official client of Carnegie Mellon University and University of Pittsburgh, among probably very few others. At its heart, it is a Mac OS (Classic) application, with an MDI user interface that is often fairly nice to use. However, the developers who ported it to Windows saw no reason to change the interface whatsoever. As a result, there are a lot of weird kludges and quirks, such as the menu bar occupying its own window (a la QuickTime 4 player) and windows not being raised simultaneously when using the Taskbar to change the current application. These quirks are not nearly as grievous when using the more well-behaved Solaris and Linux ports.
Mulberry also stores the user's Preferences on the server side, if your mail server happens to be named Cyrus. It can also allegedly check other POP and IMAP accounts, although that feature crapped out on me shortly after I started to use it. YMMV.