The problem I had with MailMate was the lack of a threaded message view - that's now a total non-starter for me.
Leaving 'View → Organize by Thread ⇧⌘T' checked and switching to 'View → Layout → Thread Arcs ⌘3' isn’t what you’re looking for?
Shown here:
http://www.macdrifter.com/2013/11/mailmate-explorations.html
Mailmate looks good but I have to confess I never gave it a shot due to it's design. Just looks so outdated.
Yeah, if you are really into aesthetics, and they are a dealbreaker, it's probably not for you. not sure how much nicer it can look though as it's a standard email client really.
Try out the demo and see if it is for you.
edit: that's his site: http://freron.com/