When I test my form, it generates a "Could not instantiate mail function" error and the form isn't sent. What's wrong?
That error message is usually generated if your Joomla Global Config mail settings are not right.
It could be that you need to use Sendmail or SMTP instead of PHPmailer (most common), or it could be that there is an error in your Email settings.
First, try changing your Global Config first to sendmail and see if the Email goes through without an error being generated. If it does, then that was the issue. If it doesn't, then try again but changing the mail setting to SMTP. If neither of those remove the error, please contact support.
