You can refer to IMAP folders, or local folders, you can use mbox or maildir local storage. And, if you need to deal with M365 mail, you can use davmail as the IMAP/Exchange gateway. My .mailcap file is set up so that if a message is HTML only mutt passes it through "elinks -dump" automatically, and I can easily view an HTML attachment in my browser if I'm so inclined. Admittedly, I've refined all this over more than a few years. I use mail on my phone and on multiple desktops/laptops, IMAP makes that possible. (Well, that, plus a little puppet configuration and unison file syncing.) I happen to get a lot of machine-generated status mail - I couldn't reasonably deal with it without mutt. I don't currently use procmail or imapfilter for my primary mailboxes, but I do have a mutt command file that I can source which does sorting into folders that I might otherwise do with procmail. I have some mailboxes that I collect mail from with fetchmail, and then read that locally.