This is a strange one so hang in there with me on the explaination:
We are an Exchange 2000 shop and our DNS and network is provided by the larger organization using BIND DNS.
Here is how our MX records are set up (the names have been change to protect the innocent):
our.domain.com MX preference = 10, mail exhanger = mailserver1.our.domain.com
our.domain.com MX preference = 20, mail exhanger = mx.domain.com
Meaning that if the server mailserver.our.domain.com were not available the mx.domain.com server would receive the email and queue it up.
This main organization does not allow any communication with domain servers over port 25. Meaning that the mailserver1.our.domain.com is not accessible by the outside world and all mail should be routed through mx.domain.com.
There is an external group trying to send us e-mail (they are an Exchange 2000 shop as well) and the email does not get delivered. After about 2 days they receive a message saying that the mailserver1.our.domain.com server was unavailable. Their logs confirm that the server was unreachable. However, it does not seem to me that their Exchange server is attempting to send through the second MX preference mail exchanger.
Is there a setting in Exchange that could account for this? Any ideas?