I believe
Mother's Mail was where the feature originated; since then,
Seagoat's Mail, Fetish, DrumsUI and ProfitUI all have duplicates of the feature. If you don't use any of those UIs and you only want the forwarding feature, you can drop the
eq2ui_mainhud_openedmail.xml file from the Seagoat package into your custom UI folder. But I will caution you that it suffers from the general snipping/trimming/juggling of UI elements that was common ten years ago.
Below is the current default window with forwarding added.
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