email filter - How do I base an outlook rule on the number of "To:" addresses?

I would like to create an Outlook rule that stops me from sending out emails that have more than ten email addresses in the "To:" line. Is this even possible? I'm using Outlook 2010 on Windows 7 64-bit.

My reasoning: I send out a distribution letter to a large number of addresses. I usually send this email to myself with the distribution list addresses in the "BCC:" line. Every once in a while I mess up and accidentally put the distribution list addresses on the "To:" line instead. This is a problem because then each email recipient will see all the addresses on the list. I want Outlook to stop me from doing this, or at least warn me before I send it out.

I also welcome suggestions on how to get around this problem.