The SMTP deferred message 'refused to talk to me: 421 4.7.0 Not allowed' typically signifies a temporary service unavailability on the receiving mail server. Potential causes include IP blocking, server overload, maintenance, greylisting, rate limiting, incorrect DNS or authentication configurations (SPF, DKIM, rDNS), exceeding sending limits, firewall issues, or the recipient server being too busy. While retrying the email later is crucial, persistent errors warrant investigation of server configurations and contacting support.