As I have seen due to various cases this error occurs. Some of the cases are:
> When you have not configured your email correctly.
> If you are using access alternate email address in your office, or the server blocking it.
> Or some problems in the ISP.
To troubleshoot this we may need few more informations, Please answer this.
Which email client you use? Are you able to recieve any emails? Since how long you are facing this Issue? Were you able to send emails before? Are you trying with your alternate email in restricted area (eg: Office or any other work place) ?