Maybe on this, I can toss an idea out and either one of you can offer comment.
In a court context, again, when you serve legal documents on someone and they're evading service or you can't track them down, there are other ways the court will let you do it if you can prove that the person actually accessed the documents or would have seen them. Sometimes they'll even be satisfied if you take an ad out in the newspaper. Some recent case law has allowed serving people on Facebook. Would you be open to some kind of a solution where you were able to demonstrate a person got the message somehow? That's why you're sending it to certain addresses. You've done your diligence to demonstrate that this person should have gotten it, given the information you have.