This really is something Microsoft missed a trick on, I mean how could they leave a simple whitelist out of the GUI, for shame. That being said I’m going to show you how to do it, they could have made it a lot easier though!

First open up the Exchange 2010 Management console, once that’s open Expand Organization Configuration and click on Hub transport.
Once the screen refreshed, in the main window click on the Transport Rules tab.

Now we’re going to create a new Transport rule that we can add domains to which we don’t want Exchange 2010 Anti Spam to scan or filter.

In my example, I’ve called mine “Allowed Domain” so I know what this Transport rule is for.  I’ll do one from scratch for you.
On the tight hand side of the Exchange Management Console Window click New Transport Rule

This will then start the Wizard

Type the name of your rule and a Comment if you feel inclined. Click Next

Tick and Select the rule When the From address contains specific words
This will populate the box below, this is where we add our whitelisted domains.

Click on Specific Words

Here you simply add the domain name of your trusted sender/s and click Ok and then Next

Once that’s done all we need to do is give it a spam rating. To do this Highlight and Tick the action called set the spam confidence level to value
Once you’ve ticked this you’ll see the window below populate and by default gives it a Confidence Level of 0  which is what we want so it won’t get scanned.

Simply click Next and Finish and you’re all done. You can always amend this Transport rule later to add other domains.

Source: http://www.itquibbles.com/add-domain-to-whitelist-in-exchange-2010/

 

There is another way to add to the whitelist

To whitelist an email address or domain in Exchange 2010 do the following:

Start the Exchange Management Shell

Specific Address:
1. $list = (Get-ContentFilterConfig).BypassedSenders

2. $list.add(“new.mail@address.com”)
Run the $list.add for each email address that you want to include.

3. Set-ContentFilterConfig -BypassedSenders $list

Entire Domain:

1. $list = (Get-ContentFilterConfig).BypassedSenderDomains

2. $list.add(“domain.com”)

3. Set-ContentFilterConfig -BypassedSenderDomains $list

Verify Change:

Get-ContentFilterConfig