Hi Friends,

We are using an Exchange 2013 in our organization. I found that i can use telnet to send mails to internal users without authentication, that is i can send mails using our CEO's mail id to any employee in our organization without authenticating, as we are using a spam filter it is being blocked for external users. Even i can send mails to internal users from: *@gmail.com.

This is a major security issue. My Exchange's receive connector is scoped for 0.0.0.0-255.255.255.255 and i have tried disabling anonymous authentication in Default Front end connector, but it stopped mail receiving from outside mail systems throwing an : (reason: 530 5.7.1 Client was not authenticated) error.

Please help me to secure the system.

You mentioned you have a spam filter, I'm assuming it is running on a relay host sitting between Exchange CAS and Internet. If this is the case, you just need to modify the scope of the default frontend receive connector on CAS so that it only accept email from the relay host. Then other internal machines will not be able to send to Exchange without authentication.