What is Unsure mode?
Bogofilter’s default configuration will classify a message as spam or non-spam. The SPAM_CUTOFF parameter is used for this. Messages with scores greater than or equal to SPAM_CUTOFF are classified as spam. Other messages are classified as ham. There is also a HAM_CUTOFF parameter. When used, messages must have scores less than or equal to HAM_CUTOFF to be classified as ham. Messages with scores between HAM_CUTOFF and SPAM_CUTOFF are classified as unsure. If you look in bogofilter.