Launchmail is a simple SMTP client intended for use on systems where sendmail is considered too dangerous. The command line interface is derived from that of blat, so you can store recipient addresses in files, but it comes with a sendmail-compatible wrapper script, so it can be used as a drop-in replacement for sendmail.
Launchmail is freely available under the GNU General Public License, Version 2 or later.
Launchmail works on at least Linux, FreeBSD, OpenBSD, NetBSD, macOS, Solaris, OpenSolaris, kFreeBSD, and GNU/Hurd. It is written in ANSI C using POSIX. So it shouldn't be difficult to get it working on other systems.
There is a README file and a manual entry:
|README||-||Introduction, Synopsis, Documentation, Download, Install, Requirements|
||-||an SMTP client with a sendmail-compatible wrapper|