Our company was having issues sending emails to certain domains. Everything was fine except for a few of these problematic domains. These emails were just sitting in my Exchange edge server's outbound queue...
The error message I was getting in the Exchange 2007 queue was this:
"451 4.4.0 Primary target IP address responded with: "421 4.4.2 Connection dropped." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternates."
After troubleshooting the issue for a while, I determined that my email server and the problematic domains were not talking nicely with EHELO. The resolution was to created a send connector that forced HELO for the problematic domains. From the exchange management shell, run the following to create the special send connector:
New-SendConnector -Name ForceHelo -AddressSpaces problemdomain.com -ForceHELO $true
After you create this connector, you can always add other problematic domains to the connector which will force HELO.