This app was mentioned in 23 comments, with an average of 1.17 upvotes
Try Maildroid. It does support S/MIME and PGP in (I believe) all the email accounts. Personally, I don't like its UI. So I never bothered to test it with multiple accounts. But who knows.... you might find it good enough.
Hands down the best email app I've ever used, works with a lot of emails too but that's a given already. It's around $20 for the app but it's free in the playstore anyway.
If you wish for a replacement to K-9 Mail, you could try MailDroid. I've switched in the last few days and it's pretty much a drop-in replacement. They also advertise their Android Wear support which allows you to reply (using voice or quick reply presets) to mails as well as forward them.
Mail Droid is the one I use. I searched the Play Store for a long time and this is the only one that handles both really well and the programmer is very responsive if you have an issue. Plus the interface is a bit customizable so you can make it look how you'd like.
K9? From my experience with K9 more often than not I was having to refresh the inboxes to get all my mail. The menu in MailDroid is very deep and encompassing. There's a level of customization that MailDroid has that K9 didn't (last I used it was about a year and a half ago). MailDroid has options for encryption and spam filtering. You can set up different filters for notifications and automation. There's a free option, just has a small ad bar at the bottom, to enjoy and it has the full feature set from the paid app, so if you're curious, give it a shot.
I've used both and maildroid is definitely better in my book. Why not give it a try? They have a free version.
Here's the link : https://play.google.com/store/apps/details?id=com.maildroid
I believe Maildroid
The icon looks like it was copied from MailDroid:
https://play.google.com/store/apps/details?id=com.maildroid&hl=en_US
I recently surveyed the available android email apps. One important criteria for me was that email should not need to be passed through a 3rd party server. May of the email apps now are actually a "service", where they collect your email on some cloud server, then deliver it to your phone. Bluemail is one example of this. I want my phone should connect directly to my email provider (gmail, yahoo mail, hotmail, etc.). I also wanted support oauth2 authentication. I also appreciate things like dark themes and compact spacing, therefore I don't like the UI of the gmail app.
My favorite email app for non-Exchange accounts was Maildroid. https://play.google.com/store/apps/details?id=com.maildroid&hl=en
For exchange accounts, I think it's hard to beat the feature set of Nine .