That's a bit silly, their whole draw is in the fact that they change your content. If you have a youtube link, they turn that into a youtube embed. If you have http://..., they change that into an anchor. There are dozens of examples of content that you can put in an email and they will transform into something more useful, and this is generally regarded as being quite valuable.
If you have a problem with posterous manipulating your content, you should probably be hosting your own content.
If you have a problem with posterous manipulating your content, you should probably be hosting your own content.