Resolving Optonline IP blacklisting issues involves a multi-faceted approach. First, identify the specific blacklist causing the problem using online tools. Given Optonline's use of Proofpoint, checking your IP's status there is crucial, and delisting should be requested if necessary. Contacting Optonline directly through abuse@ or postmaster@ is recommended. Recognize that the block might be internal to Optonline, not a public RBL. Investigate the reasons for blacklisting, addressing issues like spam complaints, invalid addresses, or compromised systems. Ensure your server configuration is correct with SPF, DKIM, and DMARC records. Monitor bounce rates and spam complaints to maintain a good sender reputation. Follow the specific delisting procedures for each blacklist (Spamhaus, Proofpoint, Microsoft) and provide detailed information on corrective actions taken. Finally, be aware that blacklisting often results from a combination of poor sending practices, and understanding these underlying issues is critical.