@KevinJ of Kansas @bsin4684 We ended up changing the address the emails came from last Wednesday, and we've not received a complaint yet. Apparently enough people must have marked the original email address as "spam" that it wasn't going through - even to the "spam" folder. NS tech support looked at the email server logs, and it shows that the provider "accepted" the email, but since it never arrived (even on my account - via Gsuite), I can only assume they just "dropped" it. Basically the "from" address had been "burned" by at least Gsuite-based (gmail) email addresses. Others might use the same algorithm as them though.