From this good doc:
http://www.cisco.com/warp/public/788/AVVID/defining-umr.html#message_distrib_list
Messages to Distribution Lists
During an Exchange outage messages to distribution lists, such as the Unaddressed Messages distribution list, appear in the UMR but are addressed to the distribution list and not to the members of the distribution list, since lists are not expanded while in the UMR. Therefore, subscribers are not able to access them until they are expanded and delivered by Exchange.