When the Messaging Gateway (SMG) license is within one week of expiring the SMG Control Center will begin sending alert emails regarding the expiring license. In some releases, however, the alerts are sent every twenty minutes rather than once per day as expected.
This issue has been addressed with the SMG 10.8.1 release.
Provided that alerts for license expiration have been enabled in Administration > Alerts, the SMG Control Center will send license expiration alerts every 24 hours.