receiving error when registering for an event

When i register for an event, i receive the following message:

A error occurred when CiviCRM attempted to send an email (via Sendmail). If you received this error after submitted on online contribution or event registration - the transaction was completed, but we were unable to send the email receipt.

This is probably related to a problem in your Outbound Email Settings (Administer CiviCRM » Global Settings » Outbound Email). Possible causes are:

Your Sendmail path is incorrect.
Your Sendmail argument is incorrect.
The mail library returned the following error message: sendmail returned error code 8

Oops. It looks like you are already registered for this event. If you want to change your registration, or you feel that you've gotten this message in error, please contact the site administrator.

I have tried sending a test e-mail from the administerciviCRM and that works just fine. but every time you register you get this error