Your problem is most likely caused by SPF or similar mechanisms. When Postfix forwards the mail to GMail, the SPF records of the original sender's domain do not match your domain and thus GMail thinks it is spam. The proper solution to this would be using Sender Rewriting Scheme (SRS). Click here for a tutorial on how to setup SRS with Postfix.
Article ID: 98, Created: 4/23/2025 at 3:16 PM, Modified: 4/23/2025 at 3:16 PM