The reason for this error to appear during form sending on published website is because PHP function "mail" is not working properly. There can be multiple reasons for that. The most common reason is that you do not have mail service running on your server (e.g., sendmail or postfix) and/or you do not have it properly configured in configuration of PHP the website is running on.

To fix this error, try to install sendmail service to your server and specify correct value for PHP parameter "sendmail_path" (by default it has value "/usr/sbin/sendmail -t -i" which might be correct in your case as well).

Alternatively, you may specify SMTP credentials in form configuration in builder. Then mail will be sent through socket instead of PHP "mail" function.