What do X-headers in mails stand for?

Question

I'm wondering what all X- headers in e-mails stand for.

• Are they really just a custom and everyone can make them up?
• Are they documented somewhere? I've quickly browsed RFC 5322. It does not seem to mention anything about custom headers.
• When or by whom were they introduced?

Thank you for shedding some light in that field :)

Answer 1

RFC822 ("Standard for the Format of ARPA Internet Text Messages") specified in sections 4.7.4 and 4.7.5 that headers beginning with "X-" would not ever be part of any standard, and thus can be used for application-specific purposes.

For what it's worth, the more recent BCP document RFC6648 ("Deprecating the 'X-' Prefix and Similar Constructs in Application Protocols") recommends that the use of the "X-" prefix be avoided in the future, as the distinction between standardized and nonstandardized headers is not well defined, and attempting to draw such a distinction fails when commonly used nonstandard headers are adopted as standards.