I’m sure kmail used to work (a number of years ago). But something happened to it and I never managed to get it to work again. I tried again recently and no luck.

Of course Thunderbird worked immediately on a basic Imap account.

And don’t even get me started on Google integration. Although Google probably shares the blame (or is responsible for most of it) given the hoops they make you jump through to (very reluctantly) allow anything to talk to their apps.