According to
this story in the Houston Chronicle, users in San Antonio, Houston and other Texas cities (perhaps even in other U.S. cities as well) were affected by an outage which has since been resolved.
The article states: "AT&T confirms the outage was resolved at 9:18 a.m. Those who are still seeing issues may want to restart their iPhones and iPads."
My wife's iPhone was affected (San Antonio, TX) and an email sent from her was delayed over an hour this morning.