There was some discussion over the past to do script monitor on the C systems via https://
In the user manual, there is
Mail Status
http://hostname/xml/status
Host Mail Status for a Specified Host
http://hostname/xml/hoststatus?hostname=host
DNS Status
http://hostname/xml/dnsstatus
Top Incoming Domains
http://hostname/xml/topin
Top Outgoing Domainsa
http://hostname/xml/tophosts
You may need to switch to https, if you admin interface uses SSL port listening.
With commandline, you can do this
curl -k -u admin:yourpassword https://hostname/xml/status
Further scripting on checking the in_recips or delivered_recips over an extended period may show there is potential problem.