TinyMCE has a habit of rewriting the URL for images, etc. However, this becomes broken when using TinyMCE to edit things like emails. The relative path is totally useless in an email because the email is not being viewed from the server like a webpage would be.
To fix this I searched the tiny_mcs.js and found that TinyMCE had two settings in the tiny_mce.js file that are boolean flags (true,false or 0,1) dealing with URL’s. The variables are relative_urls and convert_urls, and by default they are set to true, or 1.
After changing these setting to false (or zero) I have not had any further problems with using TinyMCE to edit email content.