1. A computer-implemented 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: receiving, by a security gateway, a first message from a user, the first message comprising a cookie session identifier field and a value of the cookie session identifier; rejecting, by a message filter of the security gateway, the first message based on a rejection rule, the rejection rule rejecting messages having a cookie session identifier attribute, the cookie session identifier attribute indicating that the value of the cookie session identifier is different from a previously stored cookie session identifier value; incrementing, by the learning engine, a count of the number of messages from the user received via one or more user sessions within a predetermined amount of time and rejected based on the cookie session identifier attribute; based on the count for the attribute, determining, by the learning engine, a frequency with which messages with the cookie session identifier attribute were rejected based on the rejection rule; generating, by the learning engine, an exception rule to the rejection rule in response to determining that the frequency exceeds a threshold within the predetermined amount of time; receiving, by the security gateway, a second message having the cookie session identifier attribute; and allowing, by an adaptive filter of the security gateway, the second message, responsive to the exception rule.