I am currently using Mailcow as my SMTP relay, and I am trying to implement a feature that checks for the presence of a specific X-header, specifically the X-MS-Exchange-CrossTenant-id. The goal is to relay the email only if this header is present with the value ‘123-xxxx-yyy’, otherwise, the email should be rejected.
I have attempted to achieve this functionality using both Rspamd and Sieve, but unfortunately, neither approach has been successful so far. I am reaching out to seek your expertise and guidance on the best way to accomplish this task effectively.
Could you kindly provide me with step-by-step instructions or any relevant code snippets that could help me achieve this X-header checking feature successfully?
I sincerely appreciate your time and assistance, and I’m looking forward to resolving this issue with your help.
Thank you and best regards,