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

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.