Approx 3 weeks ago we had a PC in the company with a spam bot detected. Their server had us blacklisted for about 3 days. The problem was resolved and we were whitelisted.

We do have an email firewall on the exchange server and they are white listed. I am able to receive attachments from them as long as they are not encrypted. It's only when they encrypt it is stripped.

I am thinking perhaps that number that doesn't exist on their certificates is the "private key"?