I've been using Postbox since 2012. It's a great client and while I've tried a ton of others (Apple Mail, Thunderbird, AirMail, Sparrow, Dropbox Mailbox, etc),

I've been using Postbox since 2012. It's a great client and while I've tried a ton of others (Apple Mail, Thunderbird, AirMail, Sparrow, Dropbox Mailbox, etc), I always come back to PostBox. It's stable, simple, has access to some Thunderbird extensions, and just works like I need it to. My only gripes are that development is inconsistent at best (v4 took far too long to come out, in my opinion) and they seem to focus on features that don't add much value to the client. But overall, it's a great client that I was more than happy to pay for.