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, 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: condition, then use rules conditional module, when your a ready to implement 'send emal' in your rule, 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.

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: [email protected], 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.