Upon investigation, it was determined that the flashing and consistent pop-ups in Outlook Classic were being caused by broken image links from a sender who had sent Google Drive links. This is a known issue that can only be fixed by using a web-based version of Outlook (e.g. New Outlook) or by blocking ssl.gstatic.com in the system hosts file. The latter was chosen as the most reasonable solution for the customer's workflow. Additionally, a spam calendar invite was removed, which had created a series of appointments in the customer's calendar, making it difficult to read.