All you need is the IP of the server that will be sending mail through your exchange server. Note it is a security risk to allow any one to send mail through your server as you run the risk of getting blacklisted if the external mail server you are trusting gets compromised by a spam bot.

Solution

Allow Relay from an IP with Exchange 2010 and 2007

1.From the Exchange Management Console > Server Configuration > Hub Transport > New Receive Connector.
allow relay 2010
allow relay 2010

2. Give the connector a name and select Custom > Next.
receive connector
receive connector

3. Next.
new receive connector
new receive connector

4. Add > Add in the IP address(s) or network you want to allow relay from > OK.
relay exchange
relay exchange

5. Select the 0.0.0.0 255.255.255.255 entry and click DELETE.
Warning: Leaving this entry in will make your Exchange Server an Open Relay. (Note: This does NOT mean that your default connector is an "Open Relay" as this uses "authentication"
open relay
open relay

6. Next.
ip relay exchange
ip relay exchange

7. New.
exchange new connector
exchange new connector

8. Finish.
allow relay 2010
allow relay 2010

9. Select your new connector then right click > Properties.
edit echange connector
edit echange connector

10. On the Permission Groups tab ensure "Exchange Servers" is selected.
permission groups
permission groups

11. On the Authentication Tab > Tick "Externally Secured (for example with IPSEC)." > Apply > OK.
externally secured ipsec
externally secured ipsec




Allow Relay from an IP with Exchange 2003 and 2000

1. Launch Exchange System Manager > Administrative Groups > Administrative group Name > Servers > Servername > SMTP > Right click Default SMTP Virtual Server > Properties.
Note: If you can't see administrative groups right click the top level (in this case "First Organization (Exchange)) and tick the box to show administrative groups.
default smtp server
default smtp server

2. Access Tab > Authentication > Ensure "Anonymous Access" is enabled.
anonymous access smtp
anonymous access smtp

3. Click Relay > Ensure the default of "Only the list below" is selected > Add.
allow relay 2003
allow relay 2003

4. Add in the IP addresses(s) networks or domains you want to allow relating from > OK.
allow ip to relay 2003
allow ip to relay 2003

5. OK > Apply > OK.
exchange 2000 relay
exchange 2000 relay