I've been running small mail systems for around 25 years. Just like learning to drive - lots of stuff to learn up front. Just remember to fill up every now and again and keep on top of maintenance.
I have no idea what "MUA's the that read directly off the spool" means.
I run a few Exim MTAs ie SMTP front ends. Some front MS Exchange and some front Dovecot imapd. They work fine.
No listener required, or the services, authentication and encryption that go with them. In this case I run email for myself. Email gets pushed from an Internet MTA with global addresses, to a local MTA that happens to be my workstation. I either read the email from the console, or ssh in through a jump host.