I see a lot of articles about why people don’t like the native Apple Mail app and what app they prefer to use instead. Now of course, apps are totally personal preference. The reason I use Things as my task manager of choice might be the exact same reason you don't use it. The same goes for mail apps, there are certain apps that offer features that people just fall in love with and no longer want to be without. A few years ago there was a mail app that I loved, I can’t even remember the name of it now but like most good mail apps it died a death and never came back. Since then I’ve stuck with the Apple Mail app and honestly? I really like it.Charlotte Rose, I Like Apple Mail?
I'm in the same position, Apple's Mail app does what I need of it, which is to process my email.
It doesn't integrate with loads of other applications, but I can work around that by — OK, this next part might shock some people — writing down myself what I need to do. Bonus: fewer emails left in my inbox, and less unnecessary cruft brought into other apps.
I do drop emails into DEVONthink for archiving, but only important ones, and I don't really need an add-on for that.
The only other email app I might consider switching to would be Mozilla Thunderbird, which I've used a lot in the past on both Windows and Mac environments.