Yes, with the plug-in Mail would just send SpamSieve the new messages to process, and it didn’t even know which account they were coming from. The Mail extension system is unfortunately not reliable at doing that, so we have to go the other way and have SpamSieve ask Mail which messages are new and need to be filtered. It asks Mail for accounts by name, and it turns out that some people have multiple accounts with the same name and so sometimes Mail will return the “wrong” one. There are some changes in SpamSieve 3.0.1 to detect this situation and give you a better error message.