Tom Lane writes:
In this case it was smtp.gmail.com that added that;
it's not under your control.
Since there are others in the distribution who use Fastmail,
I note that Fastmail has a hidden feature to change these headers.
According to my correspondence with a Fastmail senior support agent,
This is not documented because, at the moment, we only like to tell
people that are actually having a problem about it.
If you want to know how to avoid getting that header, you can contact
support or ask me.
Keep in mind that Received: lines that look falsified in any way
are universally treated as a sure sign of spam.
My friends comment often that my mail is marked as spam, so, upon
reading this, I thought, does the absence of the earliest received
header cause the mail to be marked as spam? Upon further though,
I concluded that, maybe it contributes, because commercial spam
processing seems so arbitrary these days, but it's not the main cause.
My mail was getting marked as spam for years before I had Fastmail
remove this received header. I suspect that the main suspicious feature
of my email is that it is not Gmail.