The error indicates that you are using a server on PHP 5.6 or above and that the local SMTP of your hosting account has an issue with the domain certificate.
Changes to PHP 5.6 require a valid, correctly issued, and correctly set up SSL certificate.
Generally, a deeper check will show an error similar to the following:
Peer certificate CN='*.server-host-name.com' did not match expected CN='your-domain-name.com'
Either use the server's Sendmail facility or to contact your web host and ask them to fix the certificate issue.
If your web host has a valid SSL certificate for the server root (the hostname of the server you are using), you may well be required to use a valid username and password along with the SMTP mailer. Something like:
... is the server hostname which the SSL certificate covers. Your web host should be able to advise you further.
IF your web host is 100% sure that there are no issues with the SSL certificate of the actual server hostname, let them know that the problem could also be due to:
• Your web host blocking the ports needed to access your SMTP provider from your server; and/or
• You have the wrong username and password to connect to your SMTP provider
Your server logs should also indicate where the problem in the server is.