mail client for Mac with these characteristics?

I'm looking for an email client for Mac, with these characteristics:

Must have:

• One that doesn't automatically mark messages as read. Only marks a message as read when I press a certain key on the keyboard.
• Preview emails without having to open them (like modern mail clients)
• Fast and robust

Nice to have:

• Support for Mac's native spelling autocorrect

Does anyone have any recommendation?

I know programming, so no problem if I'll have to configure it using code files and things like that.

I have tried Thunderbird, but it is slow and buggy.

I'm using macOS Sierra.

I would suggest taking a look at Nylas Mail. It's open source, has a wealth of plugins and themes on GitHub, and is relatively "fast and robust." As for your other criteria:

• It allows for messages to only be marked as read manually, as can be seen below:

  

• All shortcuts (including "mark as read") are completely customizable through JSON. For instance, one could have the r key mark messages as read by putting the following in one's keymap.json:

  {
      "core:mark-as-read": "r"
  }
  

• Nylas supports a dual-pane mode (similar to Apple Mail), allowing one to preview a message without having to open it in a separate window.

Unfortunately, while Nylas does have built-in spell-check, it does not use Apple's native solution.

Nylas also boasts a host of other features, including read receipts, link tracking, message snoozing, and templated quick replies. There is a Pro version, but all of these features are in the free version (which I use).

For the record, I am not affiliated in any way with Nylas—I just like their mail client.