Adding to the Spam Whitelist
Users on whitelist will be allowed through the spam filter
Go to Your WebMail Settings
Settings—Settings

Under your user settings click on the SPAM tab
You can add users to your spam whitelist to make sure that their emails don't get stuck in our spam filter. This is especially important for users that send from hotmail, gmail, yahoo mail, etc
Click Add
Type e-mail Address
You should also make sure that the box is checked at bottom that says:
Add e-mail address of original sender while sending reply