Musashi is a useful alternative to the better-known e-mail clients.
It supports multiple accounts and multiple users; sends and receives Binhex and MIME/Base64 attachments; has filtering features and more.
Overall, Musashi is a very complete e-mail package. It also supports hierarchical folders and remote mail.
System requirements: OS:System 7 or later; Extensions: MacTCP or OpenTransport; RAM: Over 2MB.