We unfortunately appeared on a couple blacklists a couple weeks ago which was rapidly resolved in a couple hours. Since that time, we are having an issue sending email to a couple external domains, but all other email is flowing correctly.
The outgoing email messages to those domains are stuck in the queue with an error of "451 4.4.0 Primary target IP address responded with: "421 4.2.1 Unable to connect." We have an Exchange 2010 server in-house. The email admin of the external domain says that they use Symantec's Anti-Spam and Virus protection and that he has whitelisted our domain and sending IP. I have tried sending an email to them via Telnet, but that ends up being stuck in the queue also.