Possibly the message does not have a text/html part. Have you tried the following in pass_mime_types:
multipart text/html text/plain
The fact that the Content-Type: header is not folded with a CR and whitespace following the semicolon is not an issue.
If adding text/plain to pass_mime_types doesn't fix this, please post a complete raw message. You can elide the actual MIME part contents, but we need to see all the Content-Type: headers and part boundaries.
** Changed in: mailman Status: New => Incomplete
** Changed in: mailman Assignee: (unassigned) => Mark Sapiro (msapiro)