You should delete the ‘User-Agent’ header from outgoing email

June 28, 2023

We all know about the HTTP User-Agent header,
which browsers and other web things send to web servers. The nominal
purpose of this is covered in RFC 9110 section 10.1.5, and it’s
not terrible, but in practice websites have abused the header for
years (if not decades) and the whole thing is a major mess (eg).
A very long time ago, some mail clients decided that they’d advertise
by adding an ‘X-Mailer’ header to email they sent, with their name
in it. Somewhat more recently, various mail clients decided that
they would do this using a ‘User-Agent’ header (sometimes in addition
to an X-Mailer header); one common example is Thunderbird.

I have come to think that this is a bad idea and that you should
configure your mail submission server to strip User-Agent (and
probabl