How do I solve "server does not allow relaying" error message

i am getting a pop up when sending an email which says "a copy has been placed in your outbox. The recipient "xxxx@xxxxxxxxxxxxxx.com" was rejected by the server because it does not allow relaying."

i Have look at the forum to find a solution, but the answers available are not working.

Apple support have been impossible to get hold of, is there anyone who could help me. Many thanks in advance