After I installed a newer version of fail2ban on the client mail server, the attackers were no longer successful at overloading the machine as it was able to keep up with changing attacking IP addresses.

      Now they are attacking our incoming mail servers, and although they are increasing the load on these machines they can not elicit login credentials from these machines as they are not setup for authentication.

     I am installing the newer fail2ban on these machines to get the loads down as with the client server.