I have an exchange server that is sending multiple spam messages

I have an old Exchange server 2003 that starts sending out garbage emails at a rate of 3 a minute.  After 10 minutes the SMTP connector stops responding and it cannot be restarted without restarting the entire system.  I have ran virus scans using Malwarebytes on all systems and can say that everything on the network is clean.  We own 5 different Public IP addresses but only have need for one.  We have been blacklisted because of this activity so I have changed the MX records to point at a new IP address.  This obviously fixes the blacklisting but we still get the surge of bad messages.  I went and deleted thousands of messages in the queue folder I have made many changes to the SMTP connector for relaying and where things stand right now is all users are at least receiving mail but all outgoing mail sits in the outgoing queues but will go if I force the connection on a specific address.  In the queue I am seeing the bad messages but I am not forcing them.  The one difference with this site is that they are not forwarding to a  smarthost they are using DNS to route address space.
Any ideas??  This server will be replaced next year.