smartmontools uses the mail command to send email. s-nail is the only package in the repos that provides /usr/bin/mail, so it can't be that easily replaced.

Oh, I thought it is possible because I read
`It is enough to simply configure S-nail if you do not want anything else,{...}`
at https://wiki.archlinux.org/title/S.M.A.R.T.#smartd.

It reads like I could use something else.

You can use something else with `-M exec /path/to/binary` in /etc/smartd.conf. See https://man.archlinux.org/man/smartd.conf.5
It would be a lot simpler if smartd supported using sendmail instead of mail, but unfortunately it doesn't. Perhaps you can open a feature request upstream.

Oh, good to know that the way exist.
Thank you.

About the upstream ticket I need to look how their archaic project management system works and if I can search for open issue tickets without creating a new account.

Edit:
Ah, I can just redirect s-nail/mail to msmtp by filling its mta variable with the path of the msmtp binary.