I have received the following completely unhelpful response from the mailing service. I have pasted in the private area the response I am thinking of sending them, subject to your approval. If you have any other suggestion, please advise.
I've done some tests, and I don't think these are sending using different From addresses. All these emails are coming from firstname.lastname@example.org
I am talking about sending emails via the form at https://www.kxxxxxxxxxxxa.com/email_test/
So, something seems to be amiss. Also, two emails from my initial attempt never made it to my account.