A rough plugin for adding action tags to Jetpack's Post By Email. Note, these only get executed when clicking publish/update in the wp-admin currently. This is a project for personal use, any my needs were to publish a draft by email quickly, and then publish via the web interface. Some additional work would be necessary to have the tags execute…

td-post-by-email.php

<?php
/*
Plugin Name: Post By Email Customizations
Plugin URI: http://tylerdigital.com
Description: Handle Jetpack Post By Email – featured images, post formats, post content
Version: 1.0
Author: Tyler Digital
Author URI: http://tylerdigital.com
*/

/**
 * Copyright (c) 2013 Tyler Digital. All rights reserved.
 *
 * Released under the GPL license
 * http://www.opensource.org/licenses/gpl-license.php
 *
 * This is an add-on for WordPress
 * http://wordpress.org/
 *
 * **********************************************************************
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 * **********************************************************************
 */

if ( !defined( 'ABSPATH' ) ) exit;

add_action( 'save_post', 'td_auto_featured_image' );
function td_auto_featured_image( $post_id ) {
  if ( ! get_post_status( $post_id=='publish' ) ) return false;
	if ( !has_tag( 'auto-thumb', $post_id ) ) return false;
	if ( has_post_thumbnail( $post_id ) ) return false;
	if ( wp_is_post_revision( $post_id ) ) return false;
	// unhook this function so it doesn't loop infinitely
	remove_action('save_post', 'td_auto_featured_image');

	$post_tags = wp_get_post_tags( $post_id );
	foreach ( $post_tags as $key => $post_tag ) {
		if ( $post_tag->name!='auto-thumb' && $post_tag->slug!='auto-thumb' ) {
			$new_post_tags[] = $post_tag->name;
		}
	}

	$args = array(
		'numberposts'    => 1,
		'order'          => 'ASC', // DESC for the last image
		'post_mime_type' => 'image',
		'post_parent'    => $post_id,
		'post_status'    => NULL,
		'post_type'      => 'attachment'
	);

	$attached_image = get_children( $args );
	if ( $attached_image ) {
		foreach ( $attached_image as $attachment_id => $attachment )
			if ( set_post_thumbnail( $post_id, $attachment_id ) ) {
				wp_set_post_tags( $post_id, $new_post_tags );
				break;
			}
	}

	// re-hook this function
	add_action('save_post', 'td_auto_featured_image');
}

add_action( 'save_post', 'td_clear_post_content' );
function td_clear_post_content( $post_id ) {
	if ( ! get_post_status( $post_id=='publish' ) ) return false;
	if ( ! has_tag( 'clear-content', $post_id ) ) return false;
	if ( wp_is_post_revision( $post_id ) ) return false;
	// unhook this function so it doesn't loop infinitely
	remove_action('save_post', 'td_clear_post_content');

	$post_tags = wp_get_post_tags( $post_id );
	foreach ( $post_tags as $key => $post_tag ) {
		if ( $post_tag->name!='clear-content' && $post_tag->slug!='clear-content' ) {
			$new_post_tags[] = $post_tag->name;
		}
	}

	if ( wp_update_post( array(
		'ID' => $post_id,
		'post_content' => '',
	) ) ) wp_set_post_tags( $post_id, $new_post_tags );

	// re-hook this function
	add_action('save_post', 'td_clear_post_content');
}

add_action( 'save_post', 'td_post_format_image' );
function td_post_format_image( $post_id ) {
	if ( ! get_post_status( $post_id=='publish' ) ) return false;
	if ( ! has_tag( 'format-image', $post_id ) ) return false;
	if ( wp_is_post_revision( $post_id ) ) return false;
	// unhook this function so it doesn't loop infinitely
	remove_action('save_post', 'td_post_format_image');

	$post_tags = wp_get_post_tags( $post_id );
	foreach ( $post_tags as $key => $post_tag ) {
		if ( $post_tag->name!='format-image' && $post_tag->slug!='format-image' ) {
			$new_post_tags[] = $post_tag->name;
		}
	}

	if ( set_post_format( $post_id, 'image' ) ) wp_set_post_tags( $post_id, $new_post_tags );

	// re-hook this function
	add_action('save_post', 'td_post_format_image');
}

?>