Overriding mail templates in theme package.

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I am trying to override the block_form_submission.php template within a theme package.

I placed the file in /packages/theme_package/mail/block_form_submission.php

This override does nothing when located there, but if I place it in /application/mail it works fine. I've seen mixed information in the forums and documentation whether it is or isn't possible.

Is this something that's possible or not?