JavaScript and PHP immediately come to mind. Ruby has a few MIME libs of a poor quality. Perl and Python have excellent MIME facilities but, as I said above, many developers simply aren't interested in learning MIME to begin with. We eliminated MIME on the receiving end earlier (see our "email routing" API) and now we've taken the next logical step: we have eliminated MIME on the other end.

Besides, email is huge. In includes at least 4 non-trivial network protocols, several storage formats and a sea of proprietary ESP-specific extensions is error codes. There is no such thing as an "email support" in any language, only bits and pieces of uneven quality, therefore the most honest and shortest answer would be "all of them".