No auto respond email sent out?

My site has this great plugin installed and everything seems to work perfectly except one critical part: I want the system send out an auto respond email when user submit a form, during my testing, no auto email was sent out?

Can anyone enlighten? I’ve already switch the email function between wp_mail() function and PHP mail() function

Actually in my testing the wp_mail() doesn’t send out email and PHP mail() does.

Thank you very much.

https://www.ads-software.com/plugins/contact-form-manager/