Yes. You can create your own function which utilizes one of the filters provided by Postie such as postie_post_before. Two short examples are included in the filterPostie.php.sample file. See also Extending Postie.
FAQ Category: Extending/Developing
Can I add special text to the title of the post when using Postie?
Yes. You can create your own function, and use the postie_post_before filter. Two short examples are included in the filterPostie.php.sample file. See also Extending Postie.