X-Mailer is an entirely voluntary header the client can add. This is different from the sender IP information is added by the server not the client Fastmail co

X-Mailer is an entirely voluntary header the client can add. This is different from the sender IP information is added by the server not the client Fastmail could strip all unnecessary headers, but I would argue they should not be modifying any content I'm telling them to send. If I add a X-Mailer header, I want them to send it.