Sometimes you want to send email or newsgroup posts with custom headers. For example, the Approved: header can be used to post to a restricted "Announce" newsgroup for a class, or you may want to mark your email with an "X-No-Arc: yes" header to prevent a mailing list archive from saving a copy.
The easy way to add a custom header to the Thunderbird email/newsgroup compose window is by adding a user.js file to your configuration directory ( .thunderbird/default//user.js ) with the following line in it:
user_pref("mail.compose.other.header", "Approved");
