Exim (v4) is a mail transport agent. exim4 is the metapackage depending on the essential components for a basic exim4 installation.

The Debian exim4 packages have their own web page, http://wiki.debian.org/PkgExim4. There is also a Debian-specific FAQ list. Information about the way the Debian packages are configured can be found in /usr/share/doc/exim4-base/README.Debian.gz, which additionally contains information about the way the Debian binary packages are built. The very extensive upstream documentation is shipped in /usr/share/doc/exim4-base/spec.txt.gz. To repeat the debconf-driven configuration process in a standard setup, invoke dpkg-reconfigure exim4-config. There is a Debian-centered mailing list, pkg-exim4-users@lists.alioth.debian.org. Please ask Debian-specific questions there, and only write to the upstream exim-users mailing list if you are sure that your question is not Debian-specific. You can find the subscription web page on http://lists.alioth.debian.org/mailman/listinfo/pkg-exim4-users