The feature set would be extremely difficult to clone via IMAP + SMTP without being confusing. One of the major features is that you can change the subject that is displayed on an incoming email, and that it won't be changed when you respond (so if someone sent you an email with "no subject" as the subject, you can change it to "foo bar foo". They will still see "no subject" when you reply, and conversations won't be broken in Outlook). That'd be very hard to clone in an arbitrary email client that didn't know how to fix it. Another one is that every unique sender gets put into a screening folder until you tell hey where to put it (inbox/paper trail/feed), and that functionality can't be cloned in a email client (because they don't know how to force you to answer that question).

Even if they found a way to make it work now, every time they had a new feature they wanted to add to their web client, they'd have to worry about making it not break Outlook/Thunderbird/$RandomEmacsMailClient, because if it does people are going to miss emails and leave the service. And since their target market is people who hate the way email works today anyway, the target market probably won't ever want to touch IMAP again if they can help it. If they want to go back, all their mail can be exported and new mail will be forwarded to their next address. It seems fine to me.

> every unique sender gets put into a screening folder

I just set up a gmail filter to have everything skip the inbox. Adding exceptions is not so convenient but it works.

> I'm also sure I could get the "tracking pixel name-n-shame" feature they're so excited about if I took some time to write some in-depth filters.

Sure, _you_ could, but how many other potential customers can?

I'll admit, the lack of IMAP is a bit of an issue for me as well, but the other features may overcome it.

I've always been interested in running my own mailserver, but whenever I've tried, it's been surprisingly difficult (getting my IP accidentally blacklisted and stuff like that). What was your experience setting up your own server like?

I use runbox.com and am very happy with it tbh, basically has everything you seem to be looking for (but you don't have to self-host which I never want to do for email).