We have been using TLS with about 20 of our vendors for a couple years and just last week we came accross one vendor that we can no longer send encrypted email to. The error we saw in the Queue was "The remote SMTP service rejected the SSL handshake because the certificate has expired." I double checked our certificate, and it is still valid. So, we called that vendor, who in turn told us that their certificate indeed was out of date, but it had been replaced. They also indicated that TLS is working correctly with all of their other clients and that we should check to see if we had cached their certificate. I didn't think Exchange would cache the certificates, and neither can I find any information about where it would cache them if it did. Any suggestions? Where it would cache them? Other things to check?