We use the news.pariyatti.org domain for these emails. If you're willing to help troubleshoot, that would be much appreciated. Otherwise I'll try to work through your recommendations, although a lot of it is very new to me so I'm not sure how far I'll be able to get. You can reach me via email at my HN username @pariyatti.org if you wish to discuss further.
I work for a small non-profit and we've had the same issue. We use SendGrid to send out our email newsletters and the open rates have fallen from ~27% to ~13% over the past month.
Does anyone here have contacts at Google who might be able to help? [Edit: I've also reached out to SendGrid support to look into this]
Looking at the data more, the drop is due to a ~73% drop in Gmail opens. Hotmail and Yahoo opens remain stable:
