When I try to train as spam, I get the spinning beach ball on the Mac Mail app. All my other devices (iPhone and iPad) are unable to connect to my mail server (which is a Mac Server mail server). Sometimes control returns but the next one I try to train as spam the same thing happens. Eventually I have to force quit mail.
I’ve noticed that periodically my devices are unable to connect to the mail server.
When I removed the SpamSieve rule, all this behavior goes away. All my devices connect normally and no more beach ball.
If what’s happening on your Mac can affect other devices connecting to the mail server, it sounds like there’s some other problem with either your server or network.
That said, to diagnose why Mail is hanging, please see this page.