When a user on Exchange 2013 sends an email to an external recipient on Exchange 2003, the sender may get the bounce back stating 451 4.4.0 SMTPSEND.SuspiciousRemoteServerError. The error indicates a communication issue between the sending and the recipient Exchange servers.

One of the causes in the incompatibility of ciphers used during message transfer (for example, for Exchange Server 2013 on Windows Server 2008 and Exchange Server 2003 on Windows Server 2003).

Example Error:



To resolve the issue, contact Support and provide a full bounce-back message with Diagnostic Information for Administrators.