How come the List-Unsubscribe email header isn't prefaced with an X?

All the customer headers I add to outgoing emails are prefaced with X-

But from all the reading I’ve done online, on how to implement the List-Unsubscribe header, nothing has mentioned using an X. Does anyone know why?


The main difference is that List-Unsubscribe is an official standard (see whereas all headers prefixed with X- are not.

They are prefixed with an X- to

  1. Make it clear they aren’t an official standard;
  2. If a proprietary header did not use the X- prefix and then after a while did become a standard, then all email sent out before the standard emerged could be in violation of the rules of the standard that did not exist at the time the email was sent, causing all kinds of havoc and confusion.

So basically, if you want to make up a header you’re completely free to do so, just prefix it with an X- so it won’t clash with a possible future official standard.

Thank you. I have just come across some official information that the X should no longer be used in email headers. I’m removing all of mine today.

I’m not sure that’s wise. That is to say, I’m not sure the receiver of such emails would know what to do with the header without the X- prefix.

The receiver of X-headers has always been me, as I include customer headers to know the memberID, emailID, etc. I have simply updated my PHP code for my email processor to look for the new header names, minus the “X-”

Did you skim the URL I sent you? The official standard, updated back in 2012, is to do away with the X-. This is their recommendation, not mine.

Right, I was thinking you were sending emails where a third party would interpret the header. In that case a one-sides change of what headers are to be used wouldn’t make sense.

If, as it turns out, you are interpreting the headers yourself somehow then sure, that’s fine.

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