the reason is the most (if not all) email services block emails sent with php mail method as they consider it as spam (probably such emails have wrong headers)

you should go to your cpanel > emal > email accounts > create a new email and get details for ssl connection (Outgoing Server): server and port
and insert them into admin settings > Email > SMTP server:port and also fill in email login and password
'SMTP secure protocol' set as 'ssl'