We have an important business contact in China whom we have communicated with by email for years. As of the past few weeks, we cannot send an email to this business successfully. We get the message delayed and then subsequent failed to deliver 4.4.7 message. We are running exchange 2003, and can email anyone else without issue, including other contacts in foreign countries. Also, we can email the important Chinese contact with a gmail account, just not through our corporate exchange.
My conclusion is that "we can email everyone else with no problem from Exchange" and "gmail gets through" = we are being blocked by a spam filter.
Is this a fair conclusion or is there somethign else I can try? Various members of our company have tried emailing the China contact and its all the same NDR. Because of the time difference, we only communicate by email; however in a gmail message we asked them to check their spam filter and they claim they did and its all set.