I use AVG AntiVirus Free Edition and have been experiencing the same problem until I turned off Web Shield and Email Shield in AVG. With these turned off I can send without a problem. I may only have to turn off the Email Shield but I haven't checked to see if it works that way. Now if I get the problem I turn off the Shield and turn back on when the message has been sent. Only a workaround but it works for me. Good luck.