Webform conditional from Civicrm checksum
I think this is possible with the below adjustments on the webform.


Add a disabled textfield on your webform and set its default value to a token or checksum value from the URL.
Now, add the condition on this element to hide email when it's value is not blank.


Update after Karing's comment - If you need to hide the field from the webform, maybe use a little css like display:none for the same.