The mail server requires authentication, which can be achieved using one of
1. Pop before smtp -- check your email for new mail before trying to send
email. This adds your IP address to a relayhosts file and allows any email to
be sent from that IP address for up to 30 minutes. After 30 minutes you have
to authenticate again.
2. Configure your email client to authenticate automatically by checking the
"my server requires authentication" box in your email configuration. You will
be able to send mail without having to "pop" your mail box first.