Barely. It was a complex setup.
We kept a central user database in MySQL, allowing people web access. Every X minutes, the email addresses from that db were exported to a flat file, for use by Majordomo. The end result was that yes, a user could post from the web or email interchangeably.
And yes, posts entered via the web interface were routed through Majordomo and the mailing lists. They appeared in two forms, depending on the subscriber’s preference; either an individual email list message, or a list digest. I don’t think we ever had any problems with DMARC. This is fairly straight forward Majordomo/mailing list stuff.
In the last 5-10 years most people preferred to not receive any email list messages at all though, and instead just followed the conversation(s) via the web interface.
In short, over the 25 years we’ve been in operation we gone from being 100% email based, to a mix of email and web, to now being almost 100% web based by user choice. That said, we very much appreciate the ability to participate via email that’s included in Discourse, for the handful of email holdouts we still have.