there are a couple of different ideas for "blank subjects" -
here is one I found...
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As you've noticed, Outlook won't allow a rule that directly specifies a
blank subject. What you have to do is define a universal rule with
exceptions that match any subject that is not blank. Start by selecting
Rules and Alerts from the Tools menu, and click on the New Rule button.
Check the option Start from a blank rule, select Check messages when they
arrive, and click on Next. Click on Next again and click on Yes to confirm
that this rule will apply to every message. Check the first action, move it
to the specified folder. In the rule description, click on the underlined
word "specified" and create a folder named "Blank Subject" to hold your
blank-subject messages, then click on Next.
Now you've reached the exceptions page. Here's where you tell the rule to
ignore all messages whose subject is not blank. Select the option except if
the subject contains specific words, and click on the underlined phrase
"specific words" in the rule description. Type the letter a and click on the
Add button, then repeat for e, i, o, u, and y, and click on OK. Then click
on Next and Finish, and you're done. Though Outlook says "words," it really
just looks for the text string anywhere, including within other words.
