Send Message on behalf of a Distribution Group
This will allow users to send mail as a distribution group thus masking their own email address.
From the Exchange Management Shell:
Add-AdPermission "DISTRIBUTION_GROUP" -user "USER_NAME" -AccessRights extendedright -ExtendedRights "send as"
DISTRIBUTION_GROUP = Name of the Distribution Group you want the user to send as.
USER_NAME = The user you will be granting this privilege to.
An alternate method is to use "Active Directory Users and Computers" (ADUC). 1) Open Active Directory Users & Computers 2) Activate "Advanced Features" in the View menu (If not already enabled) 3) Find the distribution list you want to send as (The Find option is handy for this) 4) Open the properties and go to the "Security Tab" 5) Add the user or group in question 6) Check the "Send As" permission 7) OK your way out.
References
http://blog.954network.com/2009/10/08/send-as-a-distribution-group-exchange-200/
http://exchangepedia.com/blog/2007/03/how-to-assign-sendas-right-using.html
http://www.experts-exchange.com/Software/Server_Software/Email_Servers/Exchange/Q_23001310.html