If your server installed cPanel with CloudLinux along alt-php and you want to disable php mail() function for one domain, then you can do this using the following steps :
1. Logged into your server via SSH and Edit /etc/cl.selector/php.conf file.
2. Add below lines at the top of this file:
Directive = sendmail_path
Default = /usr/sbin/sendmail -t -i
Type = list
Range = /usr/sbin/sendmail -t -i,/bin/true
Comment = Switch to /bin/true to disable php mail
3. Then, logged into client's cpanel, change the sendmail path to /bin/true using "Select PHP Version > Switch to PHP Settings".
The account will no longer be able to send via php mail.
