Would it be possible for msmtp to provide the /usr/sbin/sendmail symlink (and also have the RPM provide it)? I know it already provides /usr/lib/sendmail (via alternatives) but it seems some packages (e.g. fail2band-sendmail) require /usr/sbin/sendmail (but maybe it's their fault - not sure). Currently /usr/sbin/sendmail is provided by esmtp, ssmtp, exim, opensmtpd, postfix and sendmail packages, and I wonder whether /usr/sbin/sendmail should be handled via alternatives as well, but that would need a coordinated effort.
Ok, looking at the package again, /usr/sbin/sendmail is actually already there (and controlled via alternatives) - not sure why I didn't saw it. Anyway, what is then still missing is the Provides: on RPM level.