What’s wrong with the built-in Mail app btw? I use it in daily basis and it’s pretty good to get its job done.
It is a very fast mail client with near empty mailboxes.
I have a few small gripes with Mail.app (it occasionally claims that some messages are unread that aren't, for instance), but nothing I've seen of other mail clients has led me to think they'd ever be better for my workflow.
I believe there are some plugins that support similar features, but I have not yet found a Mail.app snoozing that is portable as well (eg: Android/webapp support).
It can also be pretty laggy (e.g. selecting multiple messages), but this has gotten a lot better in High Sierra.
I still use it though since I have yet to find an alternative that I prefer (I've only tried a few though)