.@ Tony Finch – blog

A user just reported a problem to us involving messages they received which appeared to have text missing. The sender's signature and the quote of the message that was being replied to appeared OK, but no additional text from the sender could be seen. The recipient was using Thunderbird. The messages were multipart/alternative messages sent by Outlook, with text/plain and text/html parts.

The HTML parts did in fact include the expected text, except that it was wrapped in an IE conditional comment, like the following, where the ... was the substantive content.

  <!--[if !mso]> ... <![endif]-->

Note that this is a comment so non-Microsoft HTML processors will not display its contents, so the reply appears to be missing!

Putting IE conditional comments in email is bad enough, but using them to hide the important part of the message is simply braindamaged. The bit that takes the cake is that Microsoft's own HTML to plain text converter also ignores IE conditional comments, so the text/plain alternative also omits the sender's reply!

Does anyone have a link to any Microsoft documentation about this problem? Or any other good descriptions, for that matter.