Resolving the Yahoo SMTP error 421 4.7.0 TSS04 involves a multifaceted approach addressing temporary deferrals, sender reputation, authentication, list hygiene, and engagement. The error, indicating temporary issues due to volume or complaints, necessitates consulting Yahoo's best practices documentation and support channels. Core strategies involve verifying proper authentication (SPF, DKIM, DMARC), monitoring and improving sender reputation using tools like Google Postmaster Tools, managing list hygiene by removing unengaged subscribers, and warming up new IPs by gradually increasing sending volume. Furthermore, engagement metrics, especially complaint rates, must be carefully monitored, and implementing a DMARC policy of quarantine or reject is crucial. Addressing these areas enhances deliverability and reduces the occurrence of TSS04 errors.