Email, Messaging, & Video Calls Email How to Add a Custom Header in Mozilla Thunderbird Personalize your email headers 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 on September 18, 2020 Email Yahoo! Mail Gmail Tweet Share Email Thunderbird is a popular, full-featured, free email application from Mozilla that offers many ways to customize your email experience, such as adding custom email headers. If you'd like to add personalized headers to your email messages for tagging, tracking, marketing, or other purposes, Mozilla makes it easy. Add a Custom Header to Email in Thunderbird By default, Thunderbird uses standard email headers such as From, To, Cc, and Subject. If you'd like to create custom email headers, they'll become part of the list of available header options when you compose a new message. Here's how to add custom email headers: Select Thunderbird > Preferences from the menu bar in Mozilla Thunderbird. In the General category, scroll down and select Config Editor. You'll see a warning screen explaining that continuing may void your warranty. Select I accept the risk! to continue. In the search field, type mail.compose.other.header. In the search results, double-click mail.compose.other.header. In the Enter string value dialog box, enter your custom header and select OK. Separate multiple headers with commas. For example, typing Sender:, X-Y: adds the Sender: and X-Y: headers. Select the X to close out of about:config. Select the X to close out of Preferences. Select Write to start a new message. Select the double arrow to access other types of addressing fields. Select your custom header from the drop-down menu. Your custom header is now included in the message.