Lets say our smtp server is mail.ourcompanyserver.com

If I use an e-mail client that is outside the MTA Trusted addresses (such as outlook at home), I can send an e-mail using smtp:mail.ourcompanyserver.com (our e-mail server) to any_email_address@ourcompanyserver.com.

Zimbra allows this to go through and does not reject as long as the recipient is a valid server address. It seems this is just happening because the recipient is @ourcompanyserver.com... Whenever the same external e-mail client tries to send to email@gmail.com or email@yahoo.com, it will get rejected by our server as it should...

How do I keep Zimbra from allowing mail to be relayed through it if the mentioned^ criteria is met? Or is this intended behavior?

Thanks in advance!

Zimbra Version: 7.2.0
OS: CentOS Linux 6.2