I'm facing the same problems so I create a Free API service that recognize disposable and temporary email address: Block Temporary Email
This API is 100% free and ...
OK, thanks to Clive I stopped assuming things and checked the php.ini file. Sure enough the sendmail_path option just had "/usr/sbin/sendmail".
I've updated php.ini with "sendmail_path = /usr/sbin/sendmail -t -i" and it's working beautifully.