Send mail to product owner only when the email adress is available
The email address is added to the product by hand in a custom field


I supouse that the field for entering email, is an email field (must enter a 'valid email' in format: somename@someurl.something, avioding a 'blah blah email'), if so:
then in your rule add a condition: [your_email_field] is not empy, if that condition is far away to be implemented in rules area: condition, then use rules conditional module, when your a ready to implement 'send email' in your rule, before that accion, add condition: [your_email_field] is not empty, if user entered a 'fake email', drupal will send email, but it will not go anywhare, this way you can avoid your error, and only valid emails will be delivered.