Directories (address lists) change during migration. The Microsoft Exchange Server address list is managed on a site-by-site basis for efficient administration. When a new recipient is added to a site, that information is replicated to all servers in all sites in the organization.
Note In Microsoft Exchange Server, user addresses in the existing system are considered custom recipients.
During multi-phase migration, you must make directory changes to keep mail flowing. Directories on both Microsoft Exchange Server and your existing system can be kept updated through directory synchronization, which is discussed in later chapters.
You need to update the directories on both systems each time you migrate mailboxes. For example, if you import addresses as custom recipients during the first phase of migration, you must later remove these addresses when you migrate those users' mailboxes. You should create the custom recipients in the same container as the new mailboxes, so that the Migration Wizard can automatically remove the custom recipient addresses.
You may need to replace old addresses in the existing system directory with new addresses so that old mailboxes stop receiving mail.