To diagnose a 550 error, you need to carefully read the non-delivery report (NDR) or bounce-back message. This report often includes a description of the specific issue. Common diagnostic steps include:
Verifying the recipient's email address for accuracy and validity. Checking if your IP is blacklisted using blacklist lookup tools. Reviewing the email content to ensure it doesn't trigger spam filters. Ensuring proper authentication mechanisms like SPF, DKIM, and DMARC are in place.