To Whitelist a domain in Exchange 2010 please follow these steps:

Step 1. Open the Exchange Management Shell

Step 2. Type in the following commands

$list = (Get-ContentFilterConfig).BypassedSenderDomains

$list.add("domain.com")
Where domain.com is the domain you wish to whitelist.

Step 3. Verify

Set-ContentFilterConfig -BypassedSenderDomains $list