A 550 error indicates that the mail server returning the error cannot find the address to which the message was sent(I.E. that user does not exist on the server).

If you receive a 550 error in your mail client when sending to addresses outside of your domain, this usually means that outgoing authentication is not enabled. If your mail client does not authenticate with the mail server(I.E. send it a username and password) when sending e-mail, the server will only deliver to e-mail addresses that are setup on that specific server. In order to send to outside addresses, your mail client must have outgoing(SMTP) authentication enabled.

In Outlook, you can find the outgoing authentication option in the server settings for your account. Depending on the version of Outlook, the option will be either "My server requires authentication" or "My outgoing server (SMTP) requires authentication". The outgoing authentication should be set to use the same username and password as your incoming mail.

David D.