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It is possible to add custom headers to mails your are sending with Mail.app. You need to open the Terminal.app and use the following commands.

defaults write com.apple.mail UserHeaders '{
"X-What-Happen" = "Somebody set up us the bomb.";
"X-Accept-Language" = "de, en";
}'

To read the content of your UserHeaders:

defaults read com.apple.mail UserHeaders

To delete the content of your UserHeaders:

defaults delete com.apple.mail UserHeaders

I don’t like mail.app … but what i really hate are HTML Emails. So we can turn it off!

Execute the following command in the console and all your email will now appear in plain text:

defaults write com.apple.mail PreferPlainText -bool TRUE