I have now seen a similar instance on one of my servers. I was able to get the raw garbled response from the user. I have asked to get the raw message that was sent, but unfortunately, I haven't gotten it and I suspect I won't. I think it was an attempted post sent by mistake to the LIST-confirm address. I have tried, so far without success, to duplicate this, but I will keep at it.
The raw message is attached. This one is worse than the one in the OP in that the Content-Transfer-Encoding is base64, but the content is unreadable in the raw message. It is clearly not a valid base64 encoding. I'm guessing that the original message looked similar to the one in the OP, but Microsoft Exchange garbled it by doing some attempt at base64 decoding the unencoded plain text and placing that result in the message body
-- Mark Sapiro <mark@msapiro.net> The highway is for gamblers, San Francisco Bay Area, California better use your sense - B. Dylan