WordPress has a neat feature that allows you to easily embed content from video sharing sites such as YouTube by simply writing the URL of the video you want to embed in your post, like this:
When your post is displayed, this gets replaced with an iframe which contains the content you wanted to embed, like this:
This works great in a web browser, but isn't so good in your MailPress newsletter, because most email clients don't support iframes. Your readers end up with a newsletter with a blank space instead of the embedded video content. Andre has done some experiments to address this, but nothing has seen the light of day yet. The problem isn't really MailPress's fault, but it does make newsletters with postings containing embedded videos look pretty odd at present.
As a workaround, I suggest disabling the automatic embedding so that the email reader at least sees the link to the video instead of a blank space. To do this, add the following line to the top section of your MailPress theme's _loop.php where you disable content filter plugins that aren't compatible with MailPress:
remove_filter('the_content', array(&$GLOBALS['wp_embed'], 'autoembed'), 8 );/* Dont process autoembeds */
That will stop the link turning into an invisible iframe in the email. In order to make the link clickable, you also need to install the Auto-hyperlink URLs plugin. This will make all plain text URLs in your content clickable on your website, not just in MailPress newsletters; but that's probably not a bad thing. I suggest changing the Auto-hyperlinks default settings to avoid stripping the protocol so the link looks the way it did in your editor.