OK, this is a problem I have been looking into for a few hours.
I have a 2003 AD domain with 2 domain controllers. One of which is an exchange 2003 server sp2. I had a user tell me he got a delay notice upon sending an email to a sister company of ours that has been done successfully thousands of times. I looked in the queue and saw there were several messages from several different users stuck in the queue all to the same domain, but different users. So as it stood, I could not send an email to anybody in the external domain.
First thought is of course DNS. I am able to resolve every other address to any other domain in the world. This is the ONLY domain who mail is queued for. So I figure that domain must be having problems. Well, no they are not. I am able to telnet to the external exchange server just fine from all of our other sister companies.
I then try to ping the mx record for that domain and get the host could not be resolved. This is mysterious to me since we send a bunch of email to this same company every day and all of a sudden, with absolutely no changes, it stops. I am sending email using DNS from our exchange server. So I say, its a DNS problem. Well, no its not that either. I try to ping the IP of the mx record and get a request timed out. I am able to ping that mx record from any other domain except for the one in question, so I can say with certainty that it is not a DNS issue. The external domain is not limiting a ping reply to certain IP's.
I then tried to do a tracert on the IP of their mx record. It shows that it made it out of my ISP and made it to their ISP but then drops off there. So I can say that it is not a problem on my T1 or my ISP.
Is there any advice from anyone on what else it may be?