Centralized naming is used when you want to have all incoming messages
use a uniform, centralized addressing format (e.g.,
John.Doe@example.com) which you will convert to internal
addresses, and for outbound mail have all instances of internal
addresses converted to the uniform, centralized format.b The
advantages to centralized naming include enhanced security as the
outside world does not see actual host names or user names, stability
in that mail addresses do not become outdated as internal host names
change or employees and students change accounts, and ease of use for
users in the outside world when the centralized naming scheme replaces
possibly awkward internal mailbox names with more presentable and
mnemonic mailbox names.
b Note that although a single, "central" host name is used by the outside world, this does not mean that all mail from the outside world must flow through a single, central host. For instance, on the Internet, DNS MX records may be used to equate several hosts with the central host name. Preferences may be associated with each host so equated.