Give me a single commercial email client that is realistically competing with Outlook and GMail.

That is the wrong assumption. They don't need to compete with anyone. As a commercial software they only need to make enough money to survive for their own good. As a non-commercial software they don't even need money, just enough attention to stay useable. And in both those camps there are still many members alive.

Non-commercial doesn't really matter to the thread.

Commercially, you see the occasional upstart, but inevitably they die after a few years. I used Airmail for a while but they just stagnated (they barely kept up with Gmail and struggled with Exchange) so I had to drop them.

The market for commercial desktop email clients is super-small: the intersection of the already-niche subgroups of people who will pay for email clients, people who will pay for desktop software, and people who will pay for already-commoditized software. The effort is simply not worth the small rewards, particularly compared to the impact one could have by building on existing commodity clients.

MailMate https://freron.com/

> MailMate is not the most widespread, the cheapest, or the greatest looking email client, but I also have no aspiration to MailMate ever being any of these. Instead, MailMate aspires to be the most powerful, the most flexible, the most efficient, the most standards compliant, and the most secure email client.

HEY[0] is commercial and quite promising; admittedly with its release postponed due to COVID filing it under present-continuous “is competing” is somewhat of a leap of faith on my part.

[0] https://hey.com/problems-with-email/

Try mailspring...it’s an impressive piece of oss. Free version & pro features(paid)

https://getmailspring.com