There aren't any MIME parsers that will handle this particular type of brokenness out of the box. What you could do is search for a line beginning with "--", then read the rest of that line (trimming off the first 2 dashes) to use as a boundary string.
Then, once you've got that, prepend:
string.Format ("Content-Type: multipart/mixed; boundary=\"{0}\"\r\n", boundary);
Then try parsing it again.
BTW, if you are parsing a lot of messages, I'd recommend looking at MimeKit as it is 25x faster than OpenPOP's parser and also a lot more RFC-compliant.
Hope that helps.