550 - Relaying not allowed

This error message is coming from the mail server, not from OE itself. It means that the SMTP (outgoing) mail server did not recognize you as an authorized user and so rejected your message. Most ISPs configure their SMTP servers to recognize only those whose IP address is on the accepted list, meaning that you are connected directly to that ISP. If you use multiple ISPs, you are likely to encounter this problem. The solution is to change the SMTP server for that mail account under Tools Accounts Properties Servers.
Another possible reason for this error is anti-virus software that configures OE to use a local proxy for the SMTP server. Since scanning e-mail is not necessary to protect your computer, you should disable the e-mail scan in your anti-virus product, and then re-enter the correct name for your SMTP server.