Is there a way that Nimsoft is able to attach to a mailbox perhaps through IMAP, and monitor the mailbox for incoming email that matches a criteria for generating an alarm?
Other monitoring products have a feature like this, but I am not able to easily identify if Nimsoft can do this too.
I'm not aware that any probe does this. ews_response basically does this internally, but I believe it doesn't expose such a feature for custom alerting.
In past I wrote a custom probe that did round-trip monitoring through EWS, much like the ews_response. Probably not what you're looking for, but you could write a probe to do it.
In a similar case (didn't have to do with nimsoft, though) I wrote a PS script that checks inbox and fetches zip from it and processes it. If you could implement something like that with maybe logmon or nexec, it'd take less effor than a probe..