I have just configured my 2003 Standard Edition Exchange Server. I have followed all of the instructions for setting up TLS encryption for encrypting email messages between two domains. My goal is to send and receive encrypted emails between domain A and domain B. I am able to send email messages to domain B which are received as encrypted email messages. That is great. When the users at domain B send messages to domain A, email messages are not encrypted. I spoke to the exchange administrator at domain B and he said that his configuration is correct for sending encrypted email messages using TLS. We both followed the same instruction guide.
The only difference between the two domains is that I have a Microsoft ISA server which uses a rule to publish my Exchange server. I have an incoming SMTP rule on ISAS which points to the address of my Exchange Server in my private address space.
What could be preventing domain A from receiving encrypted email messages?