Incoming TLS connections to SMG 10.5.2 and previous did not support PFS. Some agencies have required this with the threat of possible fines in certain countries.
Symantec has added support for PFS in SMG 10.5.3. Please update at your earliest convenience to take advantage of this feature. SMG 10.5.3 is available via the Updates page by clicking the "Check for updates" button if you have not received notification of it's availability.