Email, Messaging, & Video Calls Email Add a Custom Header in Mozilla Thunderbird By Heinz Tschabitscher Writer A former freelance contributor who has reviewed hundreds of email programs and services since 1997. our editorial process Heinz Tschabitscher Updated September 28, 2019 Email Yahoo! Mail Gmail Tweet Share Email Thunderbird is a popular free email application from Mozilla. It offers several ways to customize your experience with the software. By default, Thunderbird uses the From:, To:, Cc:, Bcc:, Reply-To:, and Subject: headers at the top of its emails. For most applications, that's sufficient, but you can add custom email headers if you need them. To add custom email headers, make use of a hidden setting that allows you to set up your own headers in Mozilla Thunderbird. The 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 a Custom Header to Email in Thunderbird To add custom headers for messages in Mozilla Thunderbird: Select Thunderbird > Preferences from the menu bar in Mozilla Thunderbird. Open the Advanced category. Go to the General tab. Click Config Editor. View the warning screen that appears and then click I accept the risk! Enter mail.compose.other.header in the Search field that opens. Double-click mail.compose.other.header in the search results. Enter the desired custom headers in the Enter string value dialog screen. Separate multiple headers with commas. For example, typing Sender:, X-Y: adds the Sender: and X-Y: headers. Click OK. Close the configuration editor and preferences dialog screen. You can further customize Thunderbird by using extensions and themes available from Mozilla. Like Thunderbird itself, the extensions and themes are free downloads.