Please note that we are using the core function of wordpress " wp_mail " which uses php mail() function by default.
We check a flag prarameter to check if the email was sent successfully or not .
If that fails then this error is returned .
refer :
http://prntscr.com/an3phz
we also check in many cases all the parameter Header,to,message are generated and passed successfully in the mail function but wp_mail still is not sending mail .
We are also debugging this why this is happening so .
Also can you please confirm if the
Wordpress new user registration mail to admin email is sending out or not .
also check the buddypress activation emails ?