ill post the real response in case the link do not work:
hi there.
If you want out of office messages to be sent by members of a distribution group when a message is sent to that distribution group, you must enable the "Send out of office messages to originator" option on the Exchange Advanced tab of the distribution group's Properties in Active Directory Users and Computers.
David.
ref.:
http://forums.microsoft.com/TechNet/ShowPost.aspx?PostID=603232&SiteID=17