Exchange lists each hop in a message’s transmission in the email header. This is very helpful for troubleshooting mail flow problems however it also exposes internal server names to the outside world. If you would like to keep this information from being exposed externally you can implement a feature called Header Firewall on your Internet Send Connector using the following Exchange Management Shell command:
Remove-ADPermission -id "Connector Name" -User "NT Authority\Anonymous Logon" -ExtendedRights Ms-Exch-Send-Headers-Routing
Note that this command should be entered as a single command line string.