To diagnose the 35% decline in email open rates on SendGrid, one method is to investigate whether there were any notable changes in the email campaigns at the time the drop was noticed. This could involve alterations in email design, content, subject lines, or calls to action. If a significant change occurred around that period, it might indicate that the redesign negatively affected open rates. Moreover, I would analyze the timing and frequency of the emails sent. Was there a sudden increase or decrease in frequency? Was there a change in the days or times emails were typically dispatched? Evaluating the demographic data of the target audience, such as their time zones and the best times for email engagement, could reveal potential issues. Additionally, it's essential to assess the reliability of the technical elements. This includes verifying if there were any deliverability problems, spam complaints, or if SendGrid experienced any outages or disruptions during that timeframe. Finally, it would be wise to compare the demographics of the audience for the emails that saw a decline in open rates with those that maintained theirs. Variations in these demographics could provide valuable insights into the reasons behind the drop.