I wanted to give a quick shoutout to Hey [0] while giving K-9 a potential solution. Hey is fiercely approaching email like no other email client has in decades. It is a fundamental shift in the way we interact with email. For anyone who hasn't tried it, you're missing out on what email can be. They have a free trial but it isn't free nor is it ad supported or backed by a VC, which brings me to the point about funding. You shouldn't be sorry about charging your customers or feel only if some investor would give you money all your problems would go away. I gladly pay for Hey because it solves a real problem elegantly, sure it's not perfect, I still have to have my gmail on the side as sort of a backend to my email experience, but the problems it solves are well worth paying for. So if K-9 provides real value where no other email client provides the same level of value then their users should be more than happy to pay for it, I know I would. But if it provides some esoteric feature like PGP where even those who use it don't really know why they do and most certainly would not pay for then sure maybe you do need outside funding.
While I really enjoy Hey, I think a $99 per year price tag is insane. You‘ll likely use your email for tens of years to come, so that‘s multiple thousands of dollars for a software that doesn‘t need to change. Running a mailserver isn‘t expensive, either. It could as well be an email client with a one-time payment, in my opinion. Would I pay $99 once and use it with my existing providers? Sure! Would I pay yearly and switch all my emails so that I am basically trapped inside a subscription? No...
Running email isn't expensive neither is breathing air while walking to work, but that misses the point. They had to build a product to make email less crazy that has real value and it takes massive effort and creativity.
[0] hey.com