I need to find a way in Exchange 2010 to block mail that uses an internal "FROM" address being accepted if it is received from outside of the internal network.
For example, the public domain name is domainname.com. Right now, anyone can telnet to the mail server and use firstname.lastname@example.org as the FROM address and can send mail to a known mailbox. I need to block this from being allowed if the origin is from outside of the internal domain.
However, I have some legacy devices inside the network that need to be able to send mail anonymously to the internal address of the SMTP server. For this reason, I can't just turn off or block all anonymous mail.
I'd appreciate any thoughts on the best way to accomplish this.