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Editorial Team
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Asked: 2026-06-16T21:16:13+00:00

I’m writing a Chrome extension that builds on Gmail and which needs to add

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I’m writing a Chrome extension that builds on Gmail and which needs to add several proprietary header fields to the mail header before the message is sent. My app logic goes as follows: after the user composes a message in Gmail and presses Send button, code in the content script intercepts the Click event and I create the header fields. So my question is, how do I access the mail header where I can insert the new fields? I’m looking for something like Message.setHeader() in JavaMail and Android.

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  1. Editorial Team

    You can’t.
    Gmail doesn’t have a provision for the user to add any headers other than Reply-To.

    But, if you can show how to manually add a custom header, then we can show you how to automate that with a content script.

    Until then, to send an X-header, you’ll have to use a different email client. Configure Thunderbird, for example, to use Gmail via IMAP.

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