To begin with, I would examine the user data to determine if the decline is evident across various user segments. For example, is there a reduction in 'Open' metrics in all geographic areas or just certain locations? Additionally, I would assess the change by age, gender, and occupation demographics. If I find a particular group where the decline is significant, I would explore the reasons behind it more thoroughly. This might include conducting surveys, one-on-one user interviews, or usability studies to gain further insights. For instance, if the drop is mainly in a non-English speaking country, it could point to issues related to language localization or user education. At the same time, I would review any recent product changes. Have there been updates or new features that might have affected the user experience? I would employ A/B testing or rollback methods to confirm this. After identifying the issues, I would develop and implement appropriate solutions, collaborating with the design, development, and marketing teams as necessary. Following implementation, it is essential to monitor the 'Open' metrics, user feedback, and other pertinent KPIs to evaluate the effectiveness of the solutions enacted.