Most ISPs enforce SMTP authentication to send emails which helps them reduce spam.
But, this is not supported by most Linux distributions(Centos, Debian, etc.) by default.
So, you must manually configure your mail server to enable SMTP authentication, which requires many edits in the core configuration files.
In our role as Outsourced Tech Support, we help our web hosts and their Dedicated and VPS customers setup SMTP authentication in their servers.