Just for the hell of it
I set up an iMac with the included Postfix system; mail -s subject firstname.lastname@example.org [Enter] type or copy message body [Ctrl]+[D] was a surprisingly cathartic experience.
Having said that, the bog standard Mac email client is adequate, allowing viewing "All headers" and "Raw Source" - Turning off "Load remote content" in messages helps a lot too. Whoops! Having just checked, I have 2,200 current local messages and another 15,300 archived in local mailboxes; as I am retired, I probably should sort and prune them a bit.