In my understanding, X500 address is used in intra Exchange Org mail routing.
If mailbox has multiple X500 addresses (we have much of this case due to MIIS and migration reason.), and he or she send mail to same Exchange Org recipient, which X500 addresses is written to received msg file ?
I haven't looked at it from message perspective but from how client work on this important is an address which is in legacyExchangeDn attribute - this is what is cached on the Outlook side for a client.
You have to keep all the X.500 addresses anyway.
Tomek Onyszko, memberOf Predica FIM Team (http://www.predica.pl), IdAM knowledge provider @ http://blog.predica.pl