Check the failing recipient's address format and the specific returned result. Include the correct country code and follow the selected endpoint's recipient format. Also check suppression or eligibility results if the send belongs to a campaign.
Do not assume the whole connection is broken because one recipient fails, or assume every failed recipient is banned. Compare a redacted failing example with a known valid test. If the result remains unexplained, send support the request reference and error. Avoid bulk retries or sharing a full contact list.