In at&t, how much is sms to email?

I'm in a family plan and have unlimited text messaging. I just want to know if there's a fee for every text message that I send to email via sms (Just putting the email on the To: ___) [I have a Blackjack] Cause on our recent phone bill, the email of my friend popped out in the bill. I haven't looked at the bill yet but, does someone know how much is it per sms?