Hi Ben,
I see a feature request filed to include this in the future, however currently message/content filters do not have the capability to do that.
[Field Request] Implement alerts for monitoring Delivery Queue
https://bst.cloudapps.cisco.com/bugsearch/bug/CSCzv96453/?reffering_site=dumpcr
The only workaround that I can think of at the moment would be to push status logs from the appliance to a syslog server and have third party scripts alert based off value for field ActvRcp (column number 57)
Example:
cat status.log.current | awk '{print $57}'
Thank You!
Libin Varghese