You can easily check the build options by diffing the mutt.SlackBuild
scripts. Enabling smtp doesn't "make msmtp unnecessary." Whether
or not you use msmtp as your smtp client is a matter of choice.
I've always built mutt with:
amongst other options, and still use msmtp as my SMTP client.
If you, as I, choose to use msmtp you must tell your MUA to call
it rather than sendmail, or other. In your ~/.muttrc you would have:
rather than /usr/bin/sendmail
Seems like you still need to do some homework to determine why you're
having this problem with mutt-1.5.21.
As for me, I don't even know what you mean by "automatic transmission of
video detected by motion and transmitted by mutt via msmtp", so there is
no way I can help with that issue.
starting replying while you posted ... will look at it later