One of the cool features with Exchange 2007 is the new Edge transport
role. Simply put, this is an exchange 2007 server that knows all about
your org and the people in it and who they want to receive mail from,
but doesn't belong to the domain and is designed to be stuck in the
perimeter for all the world to see.

So how does it know what's going on in the org? Simple it maintains its
own directory database (ADAM) which it then synchronises with the
internal network using a service called EdgeSync. Obviously this
requires that you open some ports to allow it t

Network adapter	Open port	Protocol
External	25/TCP	SMTP
Internal	25/TCP	SMTP
Internal	50389/TCP	LDAP
Internal	50636/TCP	LDAP
Internal	3389/TCP	RDP

To see the full Microsoft Documentation on this click here http://tinyurl.com/yy7wv8