KMail: autoresponse setup for 10+ in To: field?
 

I want to set up a rule in my KMail that, any emails coming to a specific address with 10+ people in the To: (and/or BCC: if possible) field, get an auto response saying:

"Due to SPAM and security concerns, this address does not accept mass-emails, please send such emails using the 'bcc' field of your email program"

Yes, I would still obviously be accepting them, but I get tired of the massemails and all the [reply-to-all] people.

Can anyone suggest a way to do this? I am guessing 'regular expression' but for a non-coder like me, this is anything but regular.