Downloading the Mail extension from the download page gets me a mail extension with a minimum Thingworx version of 4.0. This newly downloaded extension also gives me the same error. Is there a separate 5.0 Mail Extension that just hasn't been uploaded yet?
Also is there a list of what breaking changes there are for extensions between 4.2 and 5.0? Mail is the only one I've had issues with using the existing extensions I had installed previously.
Provided you downloaded the Mail extension from the 5.0 Downloads page, you should have the 5.0 release of the Mail Extension (disregard the minimumThingWorxVersion). I was able to import the Mail extension successfully using Java 7 (1.7.0_55) and Tomcat (7.0.54). Please verify that you are meeting (and not exceeding) our system requirements (i.e. ThingWorx does not support Java 8 or Tomcat 8).
If you're still having issues, I'll setup a meeting to troubleshoot.