How Images, Galleries, and Shortcodes Work
Out of the box Postmatic will make magic with any images or native galleries it finds in your posts. Images will be resized and displayed retina-ready following the alignment and link-to preferences of the original post.
Shortcodes for inserting media or html elements from 3rd party plugins are not supported by default. In place of the shortcode output we display the text this content is not available in email, please click here to view in your web browser. This ensures content from plugins which is not going to display well in a variety of clients does not show up in your emails.
What we do as well is whitelist known plugins that have shortcodes which generate email-safe output. For example, most of the galleries generated with NextGen Gallery work pretty well so we support those. We also support content which is output by the Types and Views plugin.
If you have a plugin or shortcode that you use regularly let us know. We'll see how it works and can probably support it.