Exchange 2007 creates named properties for x-headers during delivery. The RTM version of Exchange 2007 would NDR messages if the named property quota was reached, even if the property were only for an x-header. Service Pack 1 changed this behavior to only NDR the message if a non X-header named property could not be mapped (in other words, a property likely to contain critical data). Rollup 8 for Service pack 1 removes the ability of non-authenticated messages to consume named property IDs. Service Pack 2 goes even further.
Full Article: Exchange Team