[userbinator]

POP3 works fine with multiple clients. I've used it that way for several decades. Doesn't need any locking.

JSON is only excusable if your protocol is designed for webmail clients. Otherwise this is a case of the emperor having no clothes.

[Fnoord]

If only one client writes data or if you constantly do full sync (quite expensive on resources), yes. You can't even have different maps to sort content though. You don't follow states. POP3 just doesn't scale well in the end. If you're in 1995 and you get 3 emails a week and get really excited you got an email, then it works very well (I still hated the full syncs on dialup though). I used POP3 for several years in the 90s, even wrote my own client (a very simple protocol) Then end 90s, I used Pine and then Mutt directly in a shell on my ISP, with IMAP and Maildir; a relief! I've never used JMAP, since 1) I quit self-hosting email due to (justified, given spam) complexity of the stack and 2) I don't use Fastmail but Soverin.