SpamAssassin has to be trained before it kicks in. By default SpamAssassin will wait until 200 Spam and 200 Ham messages are "learned." To speed this delay up, reduce the number of messages required to 100 of each by adding the following lines to the /etc/mail/spamassassin/local.cf file:

 bayes_min_ham_num 100
 bayes_min_spam_num 100