Non-responsive security assurance complaint

I would be happy to publicly post the newsletter for the Pueblo Archaeological and Historical Society that apparently now is being considered spam. I posted the most recent bimonthly newsletter to the same email list that (of course with additions and deletions over the years) I have been managing over the past 20 years. Suddenly dozens and dozens of "Non Delivery" notices. 

The daily limit to Xfinity emails is supposed to be 1,000. This email list is less than 1,000. Xfinity will only send less than 100 emails at a time. Each batch in this email list is less than 100. Attachments to emails cannot be more than 25 MB. The attachment on this email is 2.9 MB

If I send the email either one-by-one or in batches of 5 each or 10 each (I did not try batches larger than 10 to actually determine the limit) there is no problem. So what is the problem??

First of all, I do not want to be labelled a spammer, and second of all I requested an explanation of what caused the spammer flag so that whatever error was inadvertently included in the newsletter could be avoided again.

The non-responsiveness is not only annoying but disrespectful and condescending. (In one case I was actually hung up on by a technician - the 6th one that I had had to explain the situation to in spite of the fact that the calls were recorded and I was told that notes were being taken.) 

I was given ticket numbers for reference and yet only one technician that I spoke to about this situation had apparently taken time to read the notes. I repeated the problem and the experiences over and over and over. 

I was told numerous times that "engineering" was determining the problem to answer my question and to remove the flag. No such luck.