I have had this happen to me every now and then.
I am not sure if its a root issue for me.
what worked for me (may or may not work for you):
- I make sure I am connected to wifi or 3g network, before sending an mms.
- if I try to send an mms, before connecting to wifi or 3g, I get that same error message every few minutes pop up on my notification bar. if this happens I go in and cancel and delete the message, then turn off and back on wifi or 3g and resend message.
- if connected and still got the message after sending mms, sometimes a picture(s) attachment would be too large, and cause the error. When this happens, then I cancel and delete the mms and compress the pics or send them separately ...which would do the trick.
- Sometimes just turning off and back on my wifi or 3g, before sending an mms also works.
.....Again, may or may not work for you. Any combination of the above has fixed the error message for me. Hopefully you get it figured out soon, it can be pretty annoying!