I'm running msmtp 1.8.3-1 on Debian buster, and it's configured and everything works except the sendmail substitutions.
I have a from %U@myhost.mydomain.com line in my /etc/msmtprc file but I can't seem to get it to work.
The documents talk about sendmail mode: how do I tell if that's enabled? My system has a symlink from /sbin/sendmail to ../bin/msmtp and running /sbin/sendmail does get an email out as expected. But there's no difference in how that works vs. msmtp from the command-line.