Toujours utile :
By default Exchange 2007 is configured to only accept SMTP email for domains it is authoritative for, and will only relay email onto other domains for authenticated local users. Usually for Outlook/OWA based clients this is entirely sufficient as even when connecting from remote locations the clients appear to be local to the Exchange Server so it is happy to relay for them. This is thanks to the connection mechanism, Outlook Anywhere and OWA both route the email data through the IIS server, or a user on a VPN will appear to be on the LAN.
This does create a problem if you need to use an alternative mail client that does not support the Outlook web protocols, in which case it will want to use SMTP to send emails. You are most likely to encounter this scenario with non-Windows/Blackberry mobile devices and "cloud" based PIM sync applications like Apple's "MobileMe". The solution is to configure your Exchange 2007 Server to accept authenticated SMTP connections and allow them to relay emails to remote domains - note that "authenticated" is essential otherwise you will turn your server into an "open relay" which will soon be abused by spammers.
There are some fundamental differences between the SMTP implementation in Exchange 2003 and 2007 that will leave you very confused if you dont know about them. The main thing is that the Exchange 2007 no longer uses the SMTP service you were familiar with but has replaced it with the Exchange Transport service, which uses "Receive Connectors" and security permissions to define who can do what. "Receive Connectors" are like the SMTP virtual servers of before, they can be defined by the networks allowed to access them, the authentication mechanisms, and the permission groups. By default all authenticated users have full send and receive permissions for all connectors, so you shouldn't have any need to edit specific permissions.
Now if you open your Exchange Management Console and navigate to the Server Configuration - Hub Transport section you should see something like this:
--> Follow the guide at Petri's website :