No direct knowledge of the tools you're using, but a google for the error methods gave me this: https://github.com/bbottema/simple-java-mail/issues/117
Could it be possible that you have multiple versions of javaMail loaded in karaf and that you are using one version in the place where you are calling the code and another where it is used (or different versions at different places in the call stack)? If so: I can see how this could make mime types not recognized when looking for a handler. Just a thought.
