How to decode email header content

Question

When open up the header or read the raw source of an email, I see header-vales in, for example, X-Cm-Senderinfo or Thread-index to be strings of encoded text. I though they are base64 but I tried converting them with online base64 decoder and didn't work, saying the string of text is not base64 encoded. I also tried decode in BinHex - no luck either. I though MIME headers are either base64 or BinHex, but neither decoding results in readable text.

How can I decode these header-values to english?

Answer 1

A lot of these headers are randomly generated, or are hashes of some part of the message content. They're not intended to have any useful information, they're just used so that if the same header is seen again later, various systems call tell that they've seen the message (or a related message) again.

For instance, I would guess that Thread-index is randomly generated; it doesn't contain any useful information itself, it's just used so the mail client can display the thread together, even if people alter the recipients or the subject.