If you are trying to test your spam assassin configuration and find that your GTUBE test email is bouncing instead of being rewritten, this is because Spam Assassin is setup globally to reject, and send a bounce message, for any email that has a spam score of over 20.0

The GTUBE test file has a score of 1000.0 so this message is well above the 20.0 threshhold and will be automatically returned to sender.