The '550 relaying denied' bounce error arises from a multitude of factors related to authorization, configuration, and reputation. Root causes include the MX server not handling mail for the recipient, DNS and MTA misconfigurations, problems with the receiving email provider, authentication failures, recipient domain policies, reverse DNS mismatches, blacklisting, incorrect HELO/EHLO configurations, improper Google Workspace and Microsoft 365 settings, and incorrect Postfix configurations. Solutions involve ensuring proper SMTP authentication, using valid reverse DNS records, utilizing SMTP relay services, avoiding blacklists, validating the 'From:' address, properly configuring hostnames, employing the correct authentication methods (e.g., TLS), scrutinizing error codes and logs, verifying MX records, checking DNS settings, and carefully reviewing platform specific configurations.