Filter: postie_subject

This filter is called after the subject has been determined. Note that the subject can be in the email subject or the email body. You could use this filter to modify the subject before categories, post format and post type are detected.

add_filter('postie_subject', 'my_postie_subject');

function my_postie_subject($subject) {
    return '[promo] ' . $subject; // force the addition of the promo category
}