cox email re: outlook express

I've been having a constant problem for several months now with our Cox email. We can receive but not send. I've changed all the ports and whatnot to the ones Cox told us to in January, and it was fine till March. The message I receive is: (0x800CCC6F) an internal error in outgoing mail. Contact ISP if it continues. server responded : 554 eastrmimpo.305.net.cox connection refused from 68.10.147.45.

Once my husband contacted Cox, he was told it wasn't on THEIR end it was OURS. And to contact the Outlook manufacturer for help. Really???? We've Googled it and it would seem we aren't the only ones having this issue. We have uninstalled, reinstalled, changed settings, created another inbox/data file, to no solution! I've had this email for years! My yahoo & gmail accounts send and receive fine. Just NOT any of my Cox. What now?