postfix, exim and the rest that drives email on the net.

Alpine is a display-oriented email client that is suitable for both the inexperienced email user as well as for the most demanding of power users. Alpine is based on the Pine® Message System, which was also developed at the University of Washington. Alpine can be learned by exploration and the use of context-sensitive help. The user experience is highly customizable through the use of the Alpine Setup command.

latrace is a glibc 2.4+ LD_AUDIT frontend. It allows you to trace library calls
and get their statistics in a manner similar to the strace utility.

Tinyproxy is a light-weight HTTP/HTTPS proxy daemon for POSIX
operating systems. Designed from the ground up to be fast and yet
small, it is an ideal solution for use cases such as embedded
deployments where a full featured HTTP proxy is required, but the
system resources for a larger proxy are unavailable.