X-apple-i-md-m |link| Today

Because x-apple-i-md-m can contain a persistent device identifier, privacy-conscious users have raised concerns. Apple has not clarified whether this header is stripped when sending through iCloud mail servers (vs. third-party SMTP).

If you are building a tool that interacts with Apple's private APIs—such as an alternative App Store or an iCloud sync tool—you cannot ignore this header. Without a valid x-apple-i-md-m value, Apple’s servers will often return a or 403 Forbidden error, even if your username and password are correct. x-apple-i-md-m

At this point, a logical question arises: Why not just use the standard Message-ID header? The answer lies in Apple’s ecosystem-specific requirements: If you are building a tool that interacts

Unlike X-Mailer , which is human-readable, is opaque and machine-only. which is human-readable

Note that removing the header will not break email functionality. It is purely informational.