As the guy above said, its probably a temporary slow-down or high capacity situation somewhere on the internet that is out of your reach. You can't do anything about it really, but it should be temporary.
That really doesn't help. Coverage, tower congestion, issues with your outbound mail server and many other factors can affect that. If every iPhone on one or more carrier was affected you'd definitely see many more active threads on the topic and probably a story on the matter.