Coexistence Manager for Notes (CMN) log shows the following warning:
2013-11-11 11:11:11,739 WARN CMN.logging [Proxy 3] - Message Number [3] on Thread [9] from Exchange (10.1.1.1:60160) to Domino (10.1.1.2:25): Failed to secure connection from the Origin Exchange server to CMN. TLS will still be used when communicating with the Destination Domino server: NotSupportedException: The server mode SSL must use a certificate with the associated private key.
Incorrect SSL certificate is set in CMN.