I've been using KMail since I gave up XFMail in 1998. No particular problems, until . . .
I've been using ProtonMail since 2018. There is a utility called ProtonMail Bridge that runs on the local machine and unencrypts the mail so that a standard client can be used. It has behaved especially flakily in the last year or so. I raised the issue with the Proton people, who said that it works fine with Thunderbird. Which it might. I don't know and am disinclined to find out.
To use the bridge, one sets the mail client -- KMail -- to point to 127.0.0.1 and port 1143. Everything else is as though pointing to an ordinary mail server. It uses imap. I store mail in maildir format.
It sends and receives mostly just fine, with a few exceptions. The first is that it frequently, though not always, lists the same message twice in my KMail inbox, with the first listing being blank and undeletable. It shows as unread. The second listing is normal in every respect.
The second KMail oddity is that if I use the "mark all messages as read" option in the inbox, it marks all those messages as *unread* on my tablet and phone, which are using he native ProtonMail Android app (under GrapheneOS, no bridge required). Likewise, if I have read a message on, say, the tablet and then read the same message in KMail, it becomes unread again on the tablet.
All of this us over ProtonVPN as well.
So I'm trying to find out what Thunderbird does that KMail doesn't, or what KMail does that Thunderbird doesn't, and how I can set up KMail so that its behavior emulates that of Thunderbird.
Any expert on mail protocols here?
Thanks in advance.