That error is a generic dont know what to do error. If you are using a
PHP script you need
PHP installed on your server, otherwise you would need a
cgi-bin and cgiemail as far as I know.
The Form Action
http://www.ashbournes.com/formmail/formmail.pl implies you are running a
cgi script. You need to make sure it is configured with the correct paths, and that you have sufficient privs to utilize it. You can check your web hosts
faq for the correct paths etc. Most of them provide a
cgi-bin directory and cgiemail so a 3rd party option may not be required.
An example of a cgiemail script can be found here
http://www.he.net/info/cgiemail/demo.html