This really is about changing legacyExchangeDN; read and you'll see. Preamble:Existing Exchange 2003/Windows 2003 organization with ~150 mailboxes and ~300 distribution groups. Migrating to Exchange 2007 in a resource forest, with linked mailboxes referring back to the Windows 2003 forest. We used IIFP 2003 to create Exchange 2007 contacts from the 2003 mailboxes. Distribution groups were harder, since IIFP just created contacts with a single e-mail address referencing the 2003 address. We needed the Ex 2007 groups to have the discrete membership that our 2003 groups had. In order to make that work, we used CSVDE to export the groups from the 2003 forest, then we stripped out the forest-specific values, then used CSVDE to import them to the 2007 forest. Because the "member" attribute was multi-valued, we couldn't get it to import, so we ended up suprpessing the member attribute in the CSVDE output. We wrote a VB program to "replicate" the groups memberships by enumerating the members on the 2003 side and adding contacts to the corresponding 2007 group by matching on the CN. So far, so good. Now I've migrated my production mailbox as a first test. The problem I'm having is when trying to reply to messages sent before the migration. Messages from before the migration still show the correct display names in the "from" field. When I actually reply to the message, though, I get an NDR saying the group doesn't exist on the server. A group with the same display name and CN does exist in every case I've tested, but I get the error consistently. If I right-click on the name in the from field and look at the Outlook properties, the display name value on the General tab doesn't show the friendly name of the group, but instead has the X500/legacyDN value of the group from 2003. If I try the same test in OWA, it just gives me a message saying the group doesn't exist. I don't get this error with individual contacts, or with messages sent after I migrated my mailbox. Since the OU heirarchy is different in the 2003 and 2007 forests, there will never be a perfect match between the two forests. As an experiment, I tried replacing the legacyExchangeDN attribute value for one of the distribution groups in the 2007 forest with the value from the 2003 forest. That fixed the problem, in that I can both reply to groups and view the complete address book data for the group. This fix is a hack, but my only other alternative is to have users remove and re-add the receipient names when replying to messages sent before the mailbox is migrated, but that would cause a mass revolt (and I wouldn't blame them). So, I seem to have a fix for this problem, but changing a system-generated attribute like legacyExchangeDN bothers me. I can't see that it breaks anything in AD or Exchange, but my analysis is probably simplistic. What could changing this attribute break? Would it be something that wouldn't show up now, but could later? Is there an alternative? I tried adding an X.500 address to the list of addresses for the group, but that doesn't work.