We cannot send mail to one particular domain. When we do, we get a 'message delayed' message from Exchange, then a #550 4.4.7 QUEUE.Expired; message expired ## message from Exchange. I can send to that domain from gmail.com. I can do a local nslookup for the MX record, and ping the host from the MX record. When I try to telnet to host.domain.com 25, I get a 220 response with all asterisks. Helo and Ehlo both trigger a response '500 What? I don't understand'
The domain did experience a firewall issue last weekend, but the sysadmin there says that the firewall has been replaced, and is working properly, and my PING tests seem to support that. He insists that there have been no changes to their mail server.
We are on Exchange 2007