The '552 5.2.0 sender rejected AUP#POL' email error signals a permanent delivery failure due to the sending server violating the recipient's Acceptable Use Policy (AUP). This stems from sender-side issues, like poor reputation or policy breaches. Cloudmark is often involved. Resolution involves determining the email provider, reviewing their AUP, authenticating emails (SPF, DKIM, DMARC), checking blacklists, and contacting the provider. New sending domains are also often associated with spammers which can trigger the errors, as well as suspicious keywords being used in the body of the emails.