Question : Email rejected : 550 relaying prohibited

I get : '550 relaying to prohibited by administrator. (Account: xxx, SMTP server: xxx, Error number 0x800ccc79).

Question 1 - is this caused by the sending email server or the receiving email server?

Question 2 - how can I fix it?

Answer : Email rejected : 550 relaying prohibited

Alternatively, you will need to authenticate your outbound SMTP connection by clicking on the servers tab and selecting My outgoing server requires authentication and checking Use the following, then add your username / password and try again.