Just noting this 2018 article from Ars Technica.

I’ve still been on my kick that the email client you use doesn’t matter that much. It’s the 20% compared to the 80% representation your habits and technique for being good at email.

But there is some counter-evidence here. Like Jonathan Gitlin saying that Inbox (RIP) was anti-helpful in cleaning house:

… like all webmail interfaces (to me at least), isn’t nearly as conducive to a good bit of spring cleaning as an actual desktop application. Automatically bundling emails into groups—Promos, Updates, and so on—kept them out of my way, so in addition to never being read they never got deleted.

What I needed was a good email client, at least to bring my work account back into control. Apple’s Mail.app has suffered from a general if nonspecific malaise for years, and so I canvassed my coworkers for suggestions. On OS X, Airmail 3 seemed to get the most recommendations, and that’s the one I went with. After it spent several hours downloading my messages, it was another day or so’s effort to sort, cull, and then mark-as-read roughly 3,400 unread emails. At the end of it, I felt a small measure of accomplishment.

In Jonathan’s case, the email client (and a day of hard labor) actually helped him get things back under control.

A lot of votes for Airmail here.