I ditched AirMail ages ago - some emails didn’t send, sync was unreliable, and the URL schemes are proprietary. @mikeschmitz recently published this excellent article at the Sweet Setup detailing the issues that iOS email clients have at the moment, which I highly recommend.
In short, after much headbanging and installing every single iOS email client under the sun, I asked myself the question, “Do I need email on my iOS device?” And I’m convinced that the answer is no. I can log into Gmail in a web browser if I’m desperate, but I only really need to check my email once per day from my macOS machine. And best of all, I can avoid the issues with incongruent URL schemes, mapping the “Archive” folder on one machine to the “All Mail” folder on another machine, poor decisions by app developers, privacy, and so on.
Now, if the developers of MailMate for macOS would be so kind to release a email app for iOS…