I've never received email from anyone via an internal Exchange server, though. All of the (Unknown Sender) emails are regular old emails. It's over 3,000 of them (out of about 7000 total) and the older I go, the more likely they are to show up that way, but it's as you say - even after moving them to Gmail, a regular IMAP client (Outlook or Thunderbird or WinMail) will correctly read the headers. Gmail won't.