When an SMG scanner is shut down / powered off for an extended length of time, no changes made to global configuration can be propagated to the scanner while it is offline. The scanner, however, does not synchronize its configuration with the stored configuration in the Control Center on startup and so changes to global configuration, policies, sender authentication, etc are not propaged to the scanner.
This is a known issue with the 10.7.3 and prior model of configuration management.
For SMG 10.7.3 and prior, a "dummy" configuration change will need to be made to force the Control Center to propagate configuration changes that occured while the scanner was offline. This is commonly effected by either changing the logging levels in Administration > Logs or by making a temporary change to some global configuration like adding a dummy domain to Protocols > Domains.
This issue will be addressed with a later release.