Implementing Gmail and Yahoo's new email requirements, specifically regarding one-click unsubscribe and email authentication (SPF, DKIM, DMARC), is crucial for maintaining deliverability. A decline in open rates and the disappearance of the one-click unsubscribe option post-implementation often indicate underlying issues such as a decreased domain/IP reputation, technical misconfigurations with List-Unsubscribe headers (including RFC compliance), authentication problems, poor list hygiene, spammy content, or unresolved feedback loop issues. Google Postmaster Tools is a vital resource for diagnosing reputation shifts and compliance status. Initial deliverability dips can also occur as systems adjust to new features, emphasizing the need for careful monitoring and adjustments.