Recently I had to identify all SMTP sources that were still sending mail via legacy servers, with a view to reconfigure them and decommission the old infrastructure.
Wrote a script that did just that for me. Download it from https://gallery.technet.microsoft.com/Find-SMTP-sources-from-48ccb9ef.
An obvious prerequisite is to have Receive connector logging enabled for some time to allow traffic to be captured.
Enjoy :-)
