If you dig into the code of postie (plugin), you’ll probably find your answer. In particular, see the filter sample files, and then trace backwards from there. From the FAQ:
Can I add special text to the body of the post when using postie?,
Yes. You can create your own function, and use the postie_post filter. Two short examples are included in the filterPostie.php.sample file
Of note – per this ticket: post by email is slated to be dropped from WP core and switched over to a plugin, so you might be better-off switching to a plugin-based solution now, rather than building your functionality around the current core post by email functionality.
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