The form processor is written in such a way that it will not allow a blank form to be sent. If a blank form is submitted, the system will issue this error:
You cannot send a blank form
If you are able to send an apparently blank form, you might think there is a problem with the system because it is allowing a blank form to be sent. The system checks for empty values. If all submitted values are empty, the system will issue the blank form error. If it doesn't issue the error, then something must be getting submitted from your form. Usually this would be a hidden field or a submit button with a value. If this is a problem for you, you can use the Pro service and set required fields.
If it is being caused by a named value on your submit button, just remove the name from the submit input. Suppose you have this:
<input type="submit" name="submit" value="Send">
...change it to this:
<input type="submit" value="Send">
You might get the blank form error when you have completed some (or all) of the fields on your form. This is caused by some (or all) of the following:
To resolve the above, run your form page through the W3C markup validator and fix all the errors. And/or use an HTML doctype and field names without spaces. NOTE: In itself there is nothing wrong with using an XHTML doctype or spaces in the form field names but when one sees this problem, there is invariably the combination of spaces in names, XHTML doctype and a form page strewn with errors. A straight HTML form page does not need an XHTML doctype.
The system will allow an empty email input to be submitted. If this is a problem for you, you can use the Pro service and set the email field as a required field. Some people perceive this as a problem but the free service has NO required fields.