Free SMS limit (using sms_fu)

I have been using SMS_fu but it seems too good to be true for production. I'm wondering, is there any sort of limit on email to sms depending on the carrier that you can send in a given amount of time? I remember reading somewhere that there is a limit to the number of free email to sms that you can send through AT&T in a given month. Does anyone know anything aobut this? I guess I can believe that it is totally free b/c the receiver is still paying, but I have googled and cannot find any concrete answers.

Depends on the carrier in my experience. Some seem to have a hard limit of "X per hour", some seem to have a similar limit but relax it if you have a good senderbase reputation, some don't seem to limit at all.

Thanks for the info. Have you ever come across some concrete numbers given by cell phone companies or is this all through experience?

No official numbers, just experience.