What metadata gets stripped by mail clients?

Many clients remove internal or sensitive metadata when you reply, forward, or download a message. Commonly stripped items include some Received lines, spam scanning annotations, internal tracking headers, and vendor specific debug fields. Clients do this to prevent data leakage of internal infrastructure details, such as internal server names or IP addresses found in those `Received` headers.

Privacy features also rewrite or hide open tracking values, link identifiers, and sometimes User Agent details. Behind the scenes, clients act like cautious harbor masters, sharing only what is needed to continue the journey while keeping internal maps and instruments out of sight.