Examples of possible extensions are listservers, autorepliers, and games. Source code is totally included. Even CGI type scripts can be added. DixieMail is Freeware - everything about it is free, even support. I do appreciate letters of thanks, though.
Source is included.
Scroll down for documentation. This software could totally destroy your computer, ruin your love life, kick your dog, and worse. So whatever you do with it is up to you. Nothing that cause the server to crash or anything, but stuff that isn't totally secure, and stuff that stopped working as of late.