This indicates that you are using a server using PHP 5.6 or above and the local SMTP of your hosting account has issue with the domain certificate. This is because PHP 5.6 requires a valid, correctly issued and set up SSL certificate.

Generally a deeper check will show an error similar to the following:

Peer certificate CN=`*' did not match expected CN=`'

The solution is to either use the servers sendmail facility, but the best solution is to contact your hosting company and ask them to fix the certificate issue.