Allow+an+email+server+to+relay+mail+through+an+exchange+server

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.

Allow Relay from an IP with Exchange 2010 and 2007
1.From the Exchange Management Console > Server Configuration > Hub Transport > New Receive Connector. 2. Give the connector a name and select Custom > Next. 3. Next. 4. Add > Add in the IP address(s) or network you want to allow relay from > OK. 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" 6. Next. 7. New. 8. Finish. 9. Select your new connector then right click > Properties. 10. On the Permission Groups tab ensure "Exchange Servers" is selected. 11. On the Authentication Tab > Tick "Externally Secured (for example with IPSEC)." > Apply > OK.

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. 2. Access Tab > Authentication > Ensure "Anonymous Access" is enabled. 3. Click Relay > Ensure the default of "Only the list below" is selected > Add. 4. Add in the IP addresses(s) networks or domains you want to allow relating from > OK. 5. OK > Apply > OK.