Open Rates: A consistent decline in open rates can indicate that subscribers are losing interest. Click-Through Rates (CTR): A drop in CTR signifies that even if subscribers are opening your emails, they are not engaging with the content. Unsubscribe Rates: An increase in unsubscribe rates can be a clear sign of list fatigue. Complaint Rates: Higher complaint rates or spam reports can signal that your emails are not well-received.