I am only stating what everyone who wants to send form data through email needs to do.
This takes you back to your original problem. I searched for your original error message and did not find any definitive cause and since no one responded to that post, no one else has apparently seen this or knows what needs to be done to fix it. One possible reason why no one responded is that most people don't use this method, there are too many external situations that prevent the mailto: action from functioning (such as the user's browser does not have an entry for a valid email client or the user uses web based email and he does not even have an email client configured on his computer.)
The cause of the error you receive can range from - the POST/GET methods have been disabled; there is a firewall that blocks the sending of the post data to the client's browser/email client; you might need to use the GET method instead; you might not be able to include any passed parameter on the end of the mailto: address, such as the subject... Most of the people that read your original post probably assumed that you have already investigated these possibilities...