Hi Dave,
You should not need to connect to a secondary cluster, cluster9a.us.messagelabs.com will not be the primary MX record for any of our customers and if it is they are configured wrong.
I have checked your IP against our internal blacklists and also checked the reputation of your IP. You are not on our blacklists but your reputation is bad. I will reset the reputation of your IP accross our infrastructure and confirm once complete.
Kind Regards
Kevin Brosnan
Tier 2 Senior Technical Support Engineer.