IP Allow List is used to add the list of IP address into Exchange server to allow computer with IP address to use exchange server for relay purpose and mail sent for the computer which is added in IP Allow list is not scan by anti-spam agent. With this mail comming from the machine with IP allow list will not be sent to spam mails or Junk mail folder. To do this need to make sure that Anti-Spam agent is intalled on the Hub server

Instruction on how install Anti spam agent. Need to make sure you run on all the Hub Transport server

1. Open powershell -> Navigate to the folder “Program Files\Microsoft\Exchange Server\Scripts”
2. Type ./install-AntispamAgents.ps1 and Enter to install the same to get the below output

3. Restart the “Microsoft Exchange Transport” service from services
4. Open the Exchange Management Console -> Organization configuration -> select Hub Transport.
5. The “Anti-Spam” tab should be visible in Hub Transport properties. Make sure that IP Allow List is enabled. If not right click and enabled the same

Instruction on How to add IP address and IP address range into allow list can be find in the below link.

http://technet.microsoft.com/en-us/library/bb125225(EXCHG.140).aspx