For a few (years?) now, there has been an increase of so called email clients that are not so. I am talking about the “value added email clients” that require a third party server/service to work. Within that category there is Nylas, Polymail, and Sparkmail, to mention a few. Although they bring an array of features not normally found on an email client, they are worthless if the third party company server(s) goes down, or have problems. They will also cease to work as soon as the company who created them goes belly up. They are not email clients per se, they are “email services clients”.
Not standard, but niche. No good.
In times when we all live on a web browser, pretty much, email clients of all types, including the fake ones above, are quickly becoming a second class citizen. After all, who wants to open an application to read/compose emails, when you already have one open that can do it as well? Yet, should I insist on using one, I would rather use a client that will only misbehave if my email provider has problems. You know, a regular IMAP one. As a macOS user, Mail.app does the job for me.