Sending email for third parties using a Certified IP address introduces potential issues with security, list hygiene, disclosure and consent.
Third-party messages sent by your domains can confuse recipients, who may mark the email as spam and negatively affect performance metrics.
Also, third-parties are not covered under your Certification contract. Return Path certifies one entity (the parent company that owns the mail program) per application. Because third parties have not applied to and been accepted by the Certification program, they cannot benefit from the service.
In some instances, Certified senders may include third-party content only if the email's subject line, Friendly From, or Envelope Sender domains belong to the Certified sender and if original content is more prevalent than third-party content.