This normally shows up only with emails from contact form or any web generated email.
Solution
1. Open /etc/httpd/conf/httpd.conf - Virtual host block for the website, remove or correct the below line
php_admin_value sendmail_path "/usr/sbin/sendmail -t -i -f email@domain.com"
You can enter e valid email in replace. The email has to be configured with the mail server on the server you are sending from.
Restart apache for the changes to take effect. Test again to make sure the issue is fully fixed.
2. Sometimes you will have this issue cause the hostname is not properly setting up. To fix this set the hostname as a fqdn and restart mail service
