It looks like you have conflicting .jar files in your userlib folder. I had the same issue when I upgraded from Email Templates to Email Connector. In my case the solution was to remove the Email Templates module and clean my userlib. In this case, I think it was removing that javax.mail .jar file from the userlib that fixed it.
In general, if you see an error like this, go into your userlib and remove older versions of duplicate jar files, keeping the ones with the highest version numbers.
Good luck!