You can add arbitrary header lines to outgoing emails.
To set up the Custom Header plugin:
Select Edit ▸ Plugins.
Enable Custom Header.
Click the Configuration tab.
You can add, edit, or remove the header fields. For every header field you add, you can specify the keys and values. The key is used as the title of the Custom Header. You can enter multiple values for the keys. You must use a semicolon to separate every value you enter.
Click New ▸ Mail Message or press Shift+Ctrl+M to open the message composer window.
Select Insert ▸ Custom Header.
In the Email Custom Header window, you can view all the defined header fields and values.
Set the values for the header fields by using the dropdown list.
Click OK.
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