In Mozilla Thunderbird it is possible to override the X-Mailer/Useragent of the e-mail client via about:config... general.useragent.enable_overrides;true general.useragent.override;(Whatever useragent you want goes here e.g.Mozilla/5.0 (Windows NT 5.2; WOW64; rv:31.0) Gecko/20100101 Thunderbird/38.0) This is useful for testing as well as privacy and fingerprinting. There should be an option in the user-interface to do this with ClawsMail as well.
I think this is already possible: - Go in the account preferences and select the "send" tab - Uncheck the "Add user agent header" box - Check the "Add user-defined header" box - Click the "Edit" button - Add "User-Agent" with whatever value you see fit.
comment #1 is correct.