I didn't have one single tutorial, nope, but more like an amalgamation of instructions from one site, ideas from other blogs and more stuff I've been adding to my .muttrc as I look to add more features that I discover.

I haven't had any issues when sending email in particular, and don't know exactly what msmtp is (is it Microsoft's Hotmail?), but I know that Exchange in particular used to cause some trouble to mutt.

Regardless, I posted a sample version derived from my own muttrc (minus email addresses) that you perhaps can compare and adapt from here: https://paste.debian.net/1025948/

I do remember bookmarking way back when something that I called "The Mutt Bible" but sadly I think it was left in one of my old computers, which I can't find right now. Interestingly, this inspires me to avoid similar problems in the future by backing up my bookmark files. I think I searched along the lines of "mutt multiple email accounts" or something, which gave me instructions on how to set account hooks.

Good luck!