Berkeley Mail was (according to def.h) developed by Kurt Shoens, dated
March 25, 1978. I very much regret that it seems impossible to include
the people that contributed within the around fifteen years of history
of BSD Mail. If you know more about this, contact me.
After the 4.4BSD release in 1993, Mail was not further developed
officially. The code that Heirloom mailx is based on contains
numerous patches from OpenBSD, NetBSD, RedHat and Debian. Namely
the NetBSD developer Christos Zoulas wrote much of it.
The maintainer and primary developer of Heirloom mailx is
Gunnar Ritter. Its development started under the name "nail" in
February 2000 and added especially the MIME code, network protocol
support, and POSIX conformance improvements. In March 2006, the
program has been integrated into the Heirloom project.
See the ChangeLog for people who have contributed to mailx.