How to Add an Arbitrary Custom Header to Email in Mozilla or Netscape

By default, Mozilla Thunderbird, Mozilla and Netscape let you set the From:, To:, Cc:, Bcc:, Reply-To: and Subject: headers in an email. For most applications, that's fine and enough, but what if you need to set the Sender: to a specific address or fancy adding some custom header (like "X-Y: Z")?

Then you can make use of a hidden setting that allows you to set arbitrary headers in Mozilla Thunderbird or Netscape. Those user-set headers show up in the list of available fields in the To: drop-down list when you compose a message, just like the other optional headers (Cc:, for example).

Add an Arbitrary Custom Header to Email in Mozilla or Netscape

To add custom headers to emails in Mozilla or Netscape:

• Open your user.js configuration file in an editor.
• Add 'user_pref("mail.compose.other.header", "{custom headers}");'.
  • Replace {custom headers} with a comma-separated list of the custom headers you want to be available. "Sender,X-Y" adds the Sender: and X-Y: headers, for example.
• Save the file and close the editor.

Unfortunately, it is not possible to specify any default values or have certain custom headers appear in outgoing emails automatically. You still have to enter the values for the headers manually when creating messages in Mozilla or Netscape.