As a very long time user I know pretty much what to expect from SS. And this isn’t expected: on highlighting a couple of false positives in the Spam folder and then selecting Train As Good (as usual), Mail will–well, not hang, exactly, but become unresponsive. No SPOD, and Activity Monitor does not go red. But it just hangs. And the more messages I have selected before choosing Train as Good, the longer it takes to clear. I use force quit and then re-open. The issue has the feel of an AppleScript issue, but what do I know? Not much.
I have the same issue and it happens with both Outlook and Apple Mail. I’ve noticed that it isn’t the Mail application that is hanging, it’s SpamSieve that is marked “Not Responding” in Activity Monitor. When this is happening, the mail programs are actually “doing something” but do not respond to keyboard input immediately but will stack commands. It happens when training SPAM and also training good Email but it is not consistent.
What makes you say that? He said that “Activity Monitor does not go red,” but you say that it shows SpamSieve as not responding. Either way, please follow the instructions on this page to submit some more information via e-mail.