Symantec Messaging Gateway Alert - The combined message queue is larger than the configured limits

Hi,

Intermittently we get this email alert from the appliance:

Subject: Symantec Messaging Gateway Alert - The combined message queue is larger than the configured limits

Symantec Messaging Gateway Alert Notification for 

======================= ALERT NOTIFICATION ================================              

The combined message queue for the following Scanners is larger than 1048576 KB


Scanner                  Combined Message Queue Size (MB)

---------------------------------------------------------------------------

Local Host               3327.6

---------------------------------------------------------------------------

I wonder what could be the cause for that problem ?

My only solution so far is to reboot the whole VM or appliance to resume email flow.