Each GeoEvent is processed independently of any others, so there would have to be a processor that did this for you. None of the built-in processors with the 10.2 release do this. However, one of the Processor samples in the SDK (Volume Control Processor) does something very similar to what you are asking for. It controls the "volume of traffic" through the processor. For example, you could configure it to only allow 1 message every 60 seconds to go through it, and use that to filter the number of GeoEvents that are sent to your email output.