alt Hacker NewsMail causes me to procrastinate so much, and I have no idea why. I use icloud everything, so I wanted my mail there as well, but after 12-13 months I switched back to FastMail because their UX is subtly better suited to quickly sorting through mail for me. And fwiw I'm afaik not using any FastMail features that Mail doesn't have.
I don't mean this in the way that Client A > Client B, but I have spent a fair amount of thought on this, and I have not been aple to pinpoint exactly what characteristics makes the difference for me, which I find to be interesting in itself.
My goal is always to have an empty inbox. So, emails that require some sort of action from my part but are not urgent I will snooze them (Gmail feature) to a later time. Email that's like "ok thanks for letting me know" and that's the end of that interaction, I will delete. Other stuff that requires more immediate attention I'll let it hang in the inbox for at most a few days and then try to act on them.
I noticed that letting mails linger in sight in my inbox is energy draining or causes me to procrastinate.