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.