Originally Posted by wallcraft
The DRM-stripping is done by a 3rd party plug-in to Calibre, look for the tools_2.4 bundle. If you add the plug-in, Calibre can import the ePub and make it DRM-free and you can then convert it to MOBI for your Kindle. Apprentice Alf's Blog is a good place to study DRM issues.
I saw your post here and downloaded the plug-in you mentioned, but it does make any changes to my epub files when I import them into Calibre - I still get the DRM error message? I have been googling looking for help with it, but have not been able to find anything.