I added Custom Post Type support to this plugin. You can grab a copy of the modified file (or the entire plugin) from my fork of Postie on GitHub.
I have not added any UI to the plugin. Instead, you can specify the Custom Post Type in the Email Subject.
To publish a post with a Custom Post Type, you simply prefix the email subject with the Custom Post Type name followed by the Custom Post Type delimiter.
For example, if I have a Custom Post Type called ‘movies’ and I wanted the title of my post to be ‘My Favorite Movie’, I would send an email with the subject ‘Movies // My Favorite Movie’.
Postie will then look for the Custom Post Type delimiter (which is ‘//’ by default, but this can be changed on line 161 of my modified postie-functions.php). If it finds the delimiter, it will then check if Custom Post Type ‘movies’ exists.
If the Custom Post Type exists, it strips out the ‘Movies // ‘ part of the subject and publishes the post with Custom Post Type ‘movies’ and the post title as everything after ‘Movies // ‘.
If the Custom Post Type is not found, or there is no delimiter found in the subject of the email, then the post will be published as post type ‘post’, i.e., the default post type (the default behavior in Postie).
This modification works with the latest version of Postie (v1.4.4 as of this post), but I will be keeping my modified copy updated as new versions of Postie are released.
I would really love if the plugin developer could include this feature in a future release of Postie. Until then, you can grab a copy of the modified file (or the entire plugin) from my fork of Postie on GitHub.