Exim, which was developed by the University of Cambridge, is a free mail transfer agent. It is flexible, but still easy to configure, and provides security via access control lists for the purpose of spam protection. The review of email contents for viruses has also been possible since version 4. Despite the monolithic design, only a small number of critical security problems have been identified over the years.
Exim is used in Debian as standard.
Exim is free software, and its source code is freely available on the project page .