I just migrated a client from Exchange 2003 to Exchange 2007. No issues with the migration. Uninstalled Exchange 2003. No issues. The only problem I am seeing now is a problem with three domains users are trying to send mail to. Mail to other domains works without issue. In all problem cases delivery fails with "421 4.4.2 Connection Dropped" errors. Outbound smtp logs show
2009-02-26T19:21:52.304Z,Outbound,08CB665E9CA0DE03,0,,18.104.22.168:25,*,,attempting to connect
It's my understanding that the remote servers are dropping the connection. When I attempt to telnet to the servers in question on port 25 from the clients server, I connect but no helo or ehlo banner is displayed. I simply get a blinking cursor and then the connection times out. When I telnet to the same servers from my own server I get the proper response.
I have checked DNS for both my clients server and the recipients server. All of it checks out both forward and reverse lookups. Outgoing mail from my clients server displays the correct FQDN name. My client servers IP is not on any blacklists.