Quote:
Originally Posted by xyzzy
I don't believe there are any restrictions on what you name the header (ignoring possible digital signature implications should you redirect the message).
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I was under the impression that there is a convention that headers which are arbitrary extensions to the 'standard' ones should have "X-" prefixes.
That appears to have been suggested in RFC822 - see:
https://tools.ietf.org/html/rfc822 though in reverse where it is stated that no future email standard will define a 'standard' header whose name starts "X-". That means that in theory adding an "X-" header won't cause a problem if the email is then forwarded or whatever.
However, there are now lots of "X-" headers added, mainly by anti-spam measures and those designed to detect email looping round and round between various servers and never being delivered, and one drawback of there being no standard naming convention for them is that you cannot be sure that a header you add won't cause a problem if it has a name conflict with someone-else's non-standard header. So, the idea of adding one's own X-headers is not perfect, but what else would you suggest?
The fact that there's a myriad of X- headers in use, without any official agreement on what they all mean - so it's a mess - is discussed in other RFCs eg
https://tools.ietf.org/html/rfc6648
The Sieve 'editheader' extension, which supports the 'addheader' command will prevent some specific standard headers from being added.