I've been on the hunt for a decent mail client ever since Opera told its faithful to kiss off when they moved to Blink. The standalone version no longer works with modern SSL since the chinese company that bought them out no longer maintains it, and I find Thunderbird... lacking.
IMHO the world could always use another fresh set of eyes on making a mail client -- and you used the magic word. Simple.
Too many of the terminal clients end up with false simplicity, they look simpler but mak the task harder. Too many GUI clients just keep tacking on features people don't want or need whilst failing to provide some basic things like in-built spam filtering.
The only reason I'm using Thunderbird right now is it actually works with all the different mail systems I log into... I'd LOVE to find something better but half of them won't even talk to my server.