The lesser and geographically uneven prevalence of mobile phone spam is attributable to geographic variation of prevalence of mobile vs non-mobile electronic communications, the higher cost (to spammers) of and technological barriers to sending mobile messages in some areas, and to law enforcement in others. Today, particularly in North America, most mobile phone spam is sent from mobile devices that have prepaid unlimited messaging rate plans. While the rate plans allow for unlimited messaging, in reality the relatively slow sending rate (on the order of magnitude of 1/s) limits the number of messages that may be sent before an abusing mobile is shut down.