This is expected behavior. Symantec Messaging Gateway does not support SSLv3 connections when FIPS mode is enabled.
When FIPS mode is turned on, even if the Require TLS encryption option is disabled, connections using SSLv3.0 and earlier are not supported. This is as per FIPS 140-2 level 1 requirements. Please see the Symantec Messaging Gateway FIPS 140-2 level 1 Deployment Guide for more information.