Change Email notification frequency for Notifying Sponsor on Device Rules

I guess you have configured "Rule Confirmation Settings" for that DPR. Each time a device has failed temporarily matching the rule, it gets deregistered and then after some time get registered again.
If this is the case, than by design an email will be sent to the sponsor each time a rouge (new or pre existing device that try to reregister) connects in the network. This option is not customizable.

You can try to use Event mapping like shown in this article to have more control on the notifications: