Comment 3 for bug 1893268

Indeed, bugs be better filed in Debian so that the package maintainer Emmanuel Bouthenot would see this (https://tracker.debian.org/pkg/msmtp).

As for the idea:

There are a lot of programs that provide the /usr/sbin/sendmail binary (or symlink), see https://packages.debian.org/search?searchon=contents&keywords=%2Fusr%2Fsbin%2Fsendmail&mode=path&suite=stable&arch=any

All of these packages would need to agree to use update-alternatives for this. Currently I suspect no package is using it (checked Postfix and Exim pacakges, neither used update-alternatives). A single package maintainer cannot decide on this alone.

Based on https://salsa.debian.org/exim-team/exim4/-/blob/master/debian/changelog#L6390-6394 there was something like this planned in 2002 but does not exists today.

While update-alternatives could have its merits, currently these packages seem to work by all providing the same metapackage and conflicting each other via it: https://packages.debian.org/stretch/mail-transport-agent