Although I know my solution is incomplete because it generates false positives, I've created a macro in Outlook which checks the HTMLbodypart for the string cid: and puts these messages in the trash can.

I check the trash before emptying it to recover false positives which do occur but less often than spam. The method suggested by John would be nice but you do need to know who you're expecting mail from. However I suppose we mostly do know. So I should add to my macro a test against my approved list of senders and, of course, probably generate my approved list of senders from what's already in my Outlook folders. Well when I have nothing to do one weekend I might get around to that.

Pete