Send As Distribution Group

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.

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