If you're looking for a blank header, you would match it with this:
^Subject:$
or if \s is a metacharacter for "whitespace" (space, tab, etc), you could use something like
^Subject:\s*$
Note that some legitimate e-mails are sent with no subject (sender was in a hurry, or forgot). The same method applies to the 822 From: header as well (note that all of these assume RFC822 headers).
For the date, something like this should work:
^Date:\s*\?\s*$
Oh, I should point out that all of this assume that your e-mail filter can use REGEX.
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