1. A method for adaptively filtering messages routed across a network by generating exception rules to rejection rules based on attributes of messages previously received and rejected, the method comprising: rejecting, by a filter of a security gateway, a first message of a first user session, the first message having an attribute identified by a rejection rule; incrementing, by a learning engine of the security gateway responsive to the rejection of the first message, a count representing the number of user sessions having one or more messages rejected based on the attribute; rejecting, by the filter, a second message of a second user session, the second message having the attribute identified by the rejection rule; incrementing, by the learning engine, the count responsive to the rejection of the second message; generating, by the learning engine responsive to determining that the count exceeds a predetermined threshold, an exception rule to the rejection rule identifying the attribute; receiving, by the filter after generating the exception rule, a third message of the first user session having the attribute; and allowing, by the filter, the third message to pass responsive to the exception rule.