Discuss the potential advantages and drawbacks in the use of an X.500 directory service in place
of DNS and the Internet mail delivery programs. Sketch the design of a mail delivery system for
an internetwork in which all mail users and mail hosts are registered in an X.500 database.

What will be an ideal response?

For access to conventional email addresses (based on Internet Domain Names), X.500 would provide a similar
facilities to the DNS service. X.500 is designed to be scalable. If this is achieved in practice, then it should
meet the future needs of large-scale networking better than DNS.
The main advantage of X.500 is that it is an attribute-based directory service. In principle, users could address
messages to people by quoting their real names and their organisational affiliations, instead of the Domain
Name based addresses currently used. The mail system would make a search request of X.500 to find the
corresponding DNS or other network address of the user’s mailbox. A drawback is that searching with a wide
scope is quite slow and costly in computing resources, the scope could be limited by the use of the
organisational affiliation. Several alternate mailboxes could be held in the directory server, providing fault
tolerant mail delivery.