From
https://www.oreilly.com/library/view/sendmail-3rd-edition/1565928393/
pr03s03.html
"The sendmail program was originally written by Eric Allman while he
was a student and staff member at the University of California at
Berkeley.
...
A sudden increase in protocol types, coupled with the anticipation of
an explosion in the number of networks, motivated Eric Allman to write
delivermail — the precursor to sendmail. The delivermail program was
shipped in 1979 with 4.0 and 4.1 BSD Unix.
...
In 1980, ARPAnet began converting from Network Control Protocol (NCP)
to Transmission Control Protocol (TCP). ... Responding to these and
other changes, Eric evolved delivermail into sendmail ...
...
The first sendmail program was shipped with 4.1c BSD (the first
version of Berkeley Unix to include TCP/IP). From that first release
to the present,[2] Eric has continued to enhance sendmail, first at UC
Berkeley, then at Britton Lee, then back at UC Berkeley, then with
InReference Inc., and now with Sendmail, Inc. The current major
version of sendmail is V8, a major rewrite that includes many bug
fixes and significant enhancements.
...
In 1992, Eric started creating a new version of sendmail to merge all
the earlier versions. V8 officially adopted most of the good features
from IDA, KJS, Sun, and HP’s sendmail, and kept abreast of the latest
standards from the Internet Engineering Task Force (IETF). In 1996,
Eric began work on V8.8 sendmail. This release continued the trend
begun with V8.7, adding many requested new features and options, and
tightening security. In 1998, V8.9 was released, continuing the
direction started by V8.8.
...
In 1999, Sendmail, Inc. was founded in Emeryville, California.
Sendmail, Inc. took over maintenance and development of the open source
version of sendmail, and began work on a commercial version.
...
"
and so on