Connection Reset - Cannot send emails to messagelabs customers

We cannot send emails to messagelabs customers, our exchange queue shows a 421 4.4.2 Connection dropped due to ConnectionReset error. I confirmed our sending IP 205.157.157.155 does not have a negative reputation and I can telnet to cluster1.us.messagelabs.com and cluster4.us.messagelabs.com from the exchange server.

How can this be cleared up? Our client cannot communicate with their bank and one of their main customers....

Thank you