I'm having the same problem. I believe its something with the network and someone told me today it could be due to the solar flares. You might wanna read some articles about the flares from this year because apparently they are the worst they've ever been and satellites have to be shutdown or else they fry some how or other.
Did you disable cellular data in an active smart action? MMS messages cannot be sent over wifi, so cellular data must be enabled. Other than that, it's probably a network issue.