It is meant to be an hourly limit. I think in your case it must be recording cumulatively.
I no longer think the counter was recording cumulatively. I think it counts multiple recipients so the total increased dramatically as a result of the total number of recipints receiving messages within the hour.
The number of recipients per meassage has now been set at 5, with an hourly limit of 10. Please let me know if this causes any problems.