|Add a filter "does not not contain" an "at" sign ("@") as all valid email addresses should contain one. Route the mail to your "Spam" mailbox.|
This technique will also filter out the obnoxious "undisclosed.recipients".
I had thought of simply deleting the message but it's too dangerous. There are valid instances when mail might be received without a "To:" entry containing an "@" - for example I am a member of a hiking club and when I coordinate hikes I inform the participants of the details via EMail. I put all their names in the "BCC:" line and generate a ficticious "TO" entry (Poco requires one). I don't typically put an "@" in this entry instead putting something like "XYZ Hike Participants".
Given that there are times when email without a "To" entry is valid it might be preferable to script this. This way you could keep the the message if it comes from a known user (one in one of your address books) and delete it if it does not. This script could be based on the "no_spam_vacation_messages" script in the registered users area.