We're having this same issue.
My sending host is: remote.airbearings.com (96.88.26.65)
mail domain is: airbearings.com
Sending to corning.com and ajg.com. (MessageLabs clients)
Remote Server at cluster4a.us.messagelabs.com (50.19.165.100) returned '441 4.4.1 Error encountered while communicating with primary target IP address: "421 4.4.2 Connection dropped due to ConnectionReset." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.
If I create a DNS zone for corning.com and ajg.com, I can force the MX records to resolve without the "a" (cluster4.us.messagelabs.com), and get email delivered to corning.com or ajg.com.
When I let the DNS queries resolve naturally, the messages eventually timeout, and an NDR is issued to the sending user.
Please check my IP for throttling/blocking, and assist if you can.
Thanks!
Brian