Cleaning an email list with steadily decreasing open rates requires a comprehensive strategy involving multiple tactics. A core element is identifying and removing unengaged subscribers (typically inactive for 3-12 months), along with invalid addresses and unsubscribes, to maintain good list hygiene and improve deliverability. Re-engagement campaigns should be used to attempt to revive interest before removal. Implementing double opt-in from the start and validating email addresses at signup helps prevent low engagement. It's also vital to address the root cause of declining open rates, considering content relevance, audience expectations, and general list hygiene practices. Segmentation, suppression lists, and sunset policies are valuable tools. Remember, fixed audiences naturally decline, so continuous subscriber acquisition is essential.