Right-click the message, select Properties, and click Details to read the message's header, which may give a clue as to its origin. If the header is too cryptic, click Message Sources to read the message itself in an inert, non-running mode (you'll see HTML code, but scroll down to view the message's text). In Outlook, to examine the headers, right-click the message and chose options; you can't view the source or text, however.