How the legacyExchangeDN and X500 address is handled in Exchane2010 and Outlook ?

In my understanding , legacyExchangeDN is used like ID in  internal Mail relay in own Exchange Org.

And it looks Outlook message include legacyExchangeDN value.(If I same msg file, it includes legacyExchangeDN)

In that case how the reply mail to orignal sender and CC user work ?

And it looks Outlook try to resolve smtp address to legacyExchangeDN value.