I believe the mail sending function is fixed on the backend. There is a code that gets all the post tags and then sets the email addresses that a post should be sent to. This was initiating the send of an email with no defined to field. Possibly because on the last go through to collect all the tags, it finds there are no more tags and then executes everything as though it needed to send an email any way. The result was that every email was sent twice – once with the appropriate information in the to field and once with nothing in the to field. This caused both attempts to fail because of a background attempt. I succeeded in stopping this perverse and stupid activity by extending the test on tags all the way to the end of the sendmail function. Now it sends just one email with the appropriate tags. I have tested with every combination of Ambassador, Cabinet and lists (from the second simulation) and all seem to be functioning ok now.