We never authenticated our domain (so our sender shows as 'via' mailchimps servers) and I think perhaps that's the next step to take. I'm worried about our open rates/deliverability taking a huge dip since our domain isn't 'warmed up' with sending that many emails out every day. Does that happen? Anything I need to do or anything else I should think about?
A fairly significant number of recipients disabling tracking/opening detection on the client end?
In my case, I have never allowed an email sender to have any knowledge of whether I have ever opened an email they send. Maybe more people are turning off the tracking?
Perhaps a Wednesday service outage occurred? I wouldn't necessarily worry about it unless it becomes a pattern.