SMTP error on checkout

HikaShop uses the standard joomla email sending functions. If you have a problem only on that email, it means that the problem is the content of the email.
Fist, you should check your different settings in the hikashop configuration under the tab mail. The encoding might not be correct for your email server, it could the ad name option , etc.
What you could try also is to send an email via the interface in sales when changing an order information. Does that work ? If yes, then, the problem is really the content of the order creation notification.
It could also be that your hosting company doesn't allow you to send more than one email per thread and at the end of the checkout, two emails are sent (1 to the customer and 1 to the vendor). So you might want to check on that with your hosting company.