Note these all apply specifically to Outlook 2010, if Outlook 2007 is still being used the directions may differ slightly.
In the Junk E-Mail folder right click on the email in question
Go to Junk
Select Not Junk
Make sure the Always trust email from "sender" is checked
Select OK
Additionally you can right click on the email in question
Go to Junk
Select Never Block Sender's Domain (@example.com)
Select OK
Additionally you can right click on the email in question
Go to Junk
Select Junk E-Mail Options
Go to the Safe Sender's Tab
Select Add
Type the domain e.g. @mail.com
Select OK
Make sure the two checkboxes marked:
Also trust e-mail from my Contacts
and
Automatically add people I e-mail to the Safe Sender's List
Are selected and press OK