Tenon Post.Office for Mac

From Tenon Intersystems:

Post.Office is a scalable SMTP/POP3/IMAP messaging server designed to meet the needs of small and mid-sized ISPs. Post.Office can support 10 users or ten thousand users on a single platform with appropriately configured hardware. Post.Office was designed from the ground up for ease of use, with an uncomplicated, user-friendly interface. The anywhere, anytime Web forms provide seamless navigation through the many Post.Office features. System administrators can make changes from anywhere on the Internet using Web forms with simple fill-in-the-blank fields and pop-up options. Postmasters can use Post.Office to easily administer enterprise email systems. End users can specify how their mail should be handled, join mailing lists, and manage mailing lists.