This may depend on the mail server the external meeting request is coming from. We recently had complaints that some external senders could not send updates to our organization. It turned out to be an Exchange version mismatch issue. Once our Exchange 2007 server received the meeting invite it made certain changes to the properties which would not allow updates to be sent by the Exchange 2003 server after that. <a href=”http://support.microsoft.com/kb/938650″>Here is the article explaining this.</a>. I know this is not the issue you described, but it may get you started in the right direction anyway. Good luck to you!