Just for your info: I saw a message that I thought should have been marked spam, and I trained it as spam manually. There was a hiccup where nothing happened for a second or two, and then the message was moved into the junk mailbox. The log registered the training and a predicted 99, and an error:
Summary: You trained a message as spam in Apple Mail.
Error Code: -1,728
Log Entry: Error moving to spam mailbox: Mail got an error: Can’t get message id 123185 of mailbox "INBOX" of account id "2649A072-5B9F-48DE-890C-2A36142740BC".
Date Logged: Today at 3:51:16 PM
SpamSieve: 3.2.2 at /Applications/SpamSieve.app
Well, I actually regard that as pretty good, because it suggests that we hit an error and recovered from the error (because we did move the message ultimately).