Subject: test Date: Sun, 17 May 2020 15:54:44 +0100 X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.32; arm-unknown-linux-gnueabihf) X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.32; arm-unknown-linux-gnueabihf) Sorry I'm not really technical, but it appears that this claws-mail sends a duplicate X-Mailer header for some reason. It is a new install on a raspberry pi 4 from the Raspbian repository.
Unless raspian patch claws-mail to do that, it doesn't seem possible. You can send a message privately if you want to provide an example.