This kind of issues may resolve with any of the following Solution;
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 you r email configuration. You will be able to send mail without having to "pop" your mail box first.