When an email that existed prior to the migration gets replied to, is it going to route using X.400 or X.500 if the email is staying within the E5.5 organization?
Exchange Migrator 1.x
Exchange Migrator 2.x
Exchange attempts to resolve the DN of the sender when replying to an existing mail message. In the case of an IntraOrg migration, the native envrionment is able to resolve the DN to the correct account because it exists in the same directory. However, if the source mailbox is deleted, Exchange will no longer be able to find the object based on DN. Therefore, the target mailbox needs to have an x500 address that contains the path to the source mailbox. In the case of an Inter Org migration the target CR and/or mailbox needs to have an x500 address that matches the DN of the source account in order for mail to be correctly delivered. If the target contacts/custom recipients are created by EMCLI DIRSYNC, EM will populate the x500 address. EM also populates migrated mailboxes with the x500 address during migration in order to facilitate mail flow.
Exchange Migrator doesn't do anything different for migrated or non-migrated objects in terms of mail flow. This is a Exchange 5.5 process that Exchange Migrator does not change.
Exchange's MTA (message transfer protocol: x.400 based component) will query the DS (directory service: x.500 based component) to determine if the object is Intra-Org or not. If it is Intra-Org, MTA then determines based on DN of the object which remote MTA to transfer. If it is Inter-ORG, MTA querys SA for route table and appropriate connectors to package and forward message to.