This is a problem introduced by Microsoft with Vista.
It is such an obvious con. Its yet another deliberate flaw and one Microsoft do not intend to fix until someone (like me) finds them out.
Microsoft claim that even after 2 years, they still cannot replicate this error - thats because they wont install Thunderbird or Lotus Notes to do it. It cannot be replicated by sending email from Outlook. This issue goes to the heart of the paranoid hostility Microsoft shows towards any company that dares to threaten its market position.
Simply put, in some email clients such as Thunderbird and Lotus Notes, if the TO: field of an email is left blank (by using BCC) the senders email client auto-populates the otherwise blank TO: field with the malformed email address "undiscosed-recipients:;" prior to sending the email. This is technically a fault with the senders email client.
The MIME decoding in Vista (used by Outlook 2003) has been changed since XP to reject this and in a manner Outlook 2003 does not expect. Outlook then simply assumes the email is in plain text ie - not MIME format.
The fix is to ask your Thunderbird or Lotus Notes user sending the BCC email to populate the TO field with their own email address. Tell them that if they leave TO: blank when sending using BCC, all Outlook/Vista recipients wont be able to read thier email. That's usually enough.
Word I have from the inside is that Microsoft will continue to try to dishonestly claim they cannot replicate this fault because they do not want to fix it. They deliberately introduced the fault in Vista as a ploy to get Outlook 2003 users to upgrade to Outlook 2005 (etc). They believe its subtle enough that it wont be put down to any deliberate act by them. Using these sorts of unethical ploys and scams are the same way Microsoft have been conning people for the last 30 years to make Bill Gates and his cronies obscenely and filthy stinking rich. They just put a few little bugs here and there to annoy the living *** out of you, and then say "hey look - its fixed in the new version!". People buy it because they have no choice - like drug addicts to a drug dealer they just keep going back again and again and again.
Wake up to it ! Demonstrate your dissatisfaction and demand tha Microsoft fix Outlook 2003 in Vista and they be prosecuted by Federal authorities over this for a monumental fraud.