We are finding each night that someone is sending emails through our Exchange server from outside. This is Exchange 2010 and I am certain that we have the open realy turned off. In the Receive connector we do NOT have externally secured checked off on the Authentication tab. I believe that is how you turn on or off an open realy. I also have set on the network tab to only receive email from our internal networks. However these emails are originating from ip addresses outside of our networks. We have a Barracuda SPAM filtering gateway so all the spam is being stopped there, however, we want to find a way to say that only email FROM our organization is permitted on the Exchange server. Last night the emails were all coming FROM email@example.com. The night before they were coming from firstname.lastname@example.org. Only email from tbj.org (our domain) should be permitted to leave our exchange server.
Can anyone help me to set this up?