This is possible per this document but the method to enable it does not appear anywhere. Help!
The procedure is available on the MEG online help. I added it below for your reference, will also update the Knowledge Base for future reference.
Task — Set message queue size alerts
Use queue size alerts to notify administrators when the number of messages in the Message Queue reaches pre-configured maximum numbers.
You can set alerts and warnings for the primary Message Queue, and also for the Inbound and Outbound queues separately. You can set thresholds for warnings, alerts or both. The warning threshold must be equal to or less than the alert threshold.
1 Log on to Email Gateway.
Email Gateway opens, displaying the Dashboard.
2 In the System Summary portlet, click the status icon beside Message Queue.
The option expands horizontally, showing data fields for warning and alert thresholds.
3 Set thresholds by typing a number to represent the required number of messages in the queue to trigger the warning or alert. Click Save.
The Message Queue option collapses to its original state.
4 Set thresholds for the Inbound or Outbound queue by clicking the Message Queue link (not the icon).
The Message Queue option expands vertically, showing the Inbound and Outbound options.
5 Set thresholds for Inbound, Outbound, or both by typing a number to represent the required number of messages in the queue to trigger the warning or alert. Click Save
The Inbound and/or Outbound queues collapse to their original state.
Hope this helps.