I'm having a problem similar to this. In short, whenever I select an EPiServer CMS page to use as HTML content in EPiServer Mail, the preview warns me "No HTML content found", and if I send the mail anyway it arrives empty. The content page is published in all available languages, and the server can resolve the site URL, so that's not the problem.
I'm running CMS 5 R2, EPiServer Mail 4.4.343.20 on IIS7.
Have you solved this issue yet? If not, You can open the host file which located in the following directory C:\Windows\system32\etc\drivers\
then add this line: 127.0.0.1 sitename
Hope this could help you!
I have the same problem and this on my development machine where the site has the url http://localhost, so edit the host file is not a solution.
Could I see your host file? Have you checked the access rights on the target pages which should be granted "everyone" role? Unpublised pages also cause this problem.
Do you get the same error if you select "url" instead of "episerver cms" in the html content tab?
@Huy Thanks for the tip about access rights, that solved the problem. Our site is an intranet so Everyone doesn't have access anywhere, until now.
@Peter Yep, but that was probably because of the access rights since it works now after setting read access for Everyone.