Smart folders don't return emails or contacts, even though the Spotlight menu search does. To work around this limitation, open the Smart Folder in Property List Editor (or a text editor) and edit the "RawQuery" string, removing this text:
Note that the text is one continuous line; it's been shown here on two lines for a narrower display. You can also modify this string to build complex boolean queries that the GUI doesn't support, as has been discussed in other hints...
&& (kMDItemContentType != com.apple.mail.emlx) && (kMDItemContentType != public.vcard)