I use monitor, with defined thresholds for notification purposes, I just don't do the external domain thing!

To correlate an e-mail delivery take a look at the POA logs (I normally leave mine in verbose mode) and you should see the e-mail being handed off from the MTA, which receives it from the GWIA, to the Post Office. It will show sender and recipient and the delivery to the said person's mailbox. If the person has blacklisted the address then you will see that in the logs and the report that the mail has been discarded.

Hope that helps you a little