I have a friend who is running on Mac OS X 10.11.6 . He has over 65,000 messages in his inbox, many of which are SPAM. If he runs SpamSieve will SpamSieve go through all his existing messages in his inbox and filter out the SPAM mail that already exists in his in box? Or does SpamSieve only filter out new SPAM emails going forward?
By default, SpamSieve only filters new messages that arrive after it was installed.
If desired, you can select a batch of old messages (Mail may get overloaded if you choose all 65K at once) and choose Message ‣ Apply Rules to have it go through them. Before doing batch processing like this, you should uncheck Auto-train with incoming mail in the preferences.