[Evolution-hackers] Why X-Mailer instead of User-Agent?

["Garreau\, Alexandre" <galex-713 galex-713 eu>]

Hi,

Recently I received a mail from Evolution, and as I recalled it a bit
(it was my first ever free-software mail user-agent! ^^ which I always
recommanded at the time I still used GNOME), I subsequently retried it
to inspect its behavior.

In both occurence I noted the usage of, non-standard as far as I
understood, “X-Mailer” header rather than the, standard as far as I
understood, “User-Agent” one [1]. Why so?  Is there a reason, or did
just this stayed this way since 2009 (just learned first version was
back in 2000, what a respectable ancestor! :D)?

[1] https://tools.ietf.org/html/rfc5536#section-3.2.13