If your Campaign's Preview button hasn't been setup up just right, you might sometimes see a webpage that says something like: "Page Not Found" with the Force.com logo on it. Here's a few things you can try to make sure the proper preview of your email appears instead.
1) Ensure that your Preview button URL is correct by going to Setup > App Setup > Customize > Campaigns > Buttons and Links > Custom Buttons and Links > Preview > Edit > the big text area at the bottom of the page should have: <<Your New Site URL>>/apex/sbm__sbx_emailviewbodyhtml?id={!Campaign.Id}
NOTE: Replace “<<Your New Site URL>> with your Site URL, INCLUDING the "/track" at the end of it.
So, if your Site URL is "http://picnet.force.com/track", your URL in this big text area would be:
http://picnet.force.com/track/apex/sbm__sbx_emailviewbodyhtml?id={!...
Very often, people just forget to include the "/track" before the "/apex..." or simply have made a mistype in the URL itself. Additionally, many times people put only one underscore between "sbm" and "sbx" in the URL, when it needs two.
Happy emailing!