Herein lies the problem, at least in terms of me being a customer of Hey. Hey is a standalone service. Unlike a typical email provider it doesn’t expose an IMAP interface which other apps can use. If it did this, it would probably lose all the pretty add-ons they have built into the system. Furthermore it doesn’t allow the use of a custom domain.
Ignoring the whole drama that is currently going on, and for which I hope Apple gets punished, Hey.com is still an interesting service that kind off works like I use email in general.
But the fact that I can't use custom domains as of right now is the reason I will not switch. And even if they did support them, I have to be sure that I can use aliases. I have around 20 email-addresses that either all forward to one address (but are different for different services) or are sorted into folders automatically. I use Fastmailby the way to achieve all of this, and yes that's a referral link. You get ten per cent off using that.
I still do like what they did though, so that guide by Andrew Canion replicating most of their feature-set is something I have to look into.
Unfortunately, there doesn't seem to be an iOS email-app that doesn't look like shit AND still supports advanced features like Preside.