Dear M. DeWitt,

The question you ask -- is X.400 better than TCP-IP -- is prone to starting a
"flame war". Arguments can easily become very religious, and several authors
are on record with a set of somewhat extreme opinions, e.g:

 * the reason X.400 is so great is that it comes in two incompatible modes,

 * when two X.400 users want to exchange mail, they take the telephone.

To put it briefly and mildly, X.400 was first developped in 1980-1984 by a
CCITT working group, with a "clean slate" approach. The theoretical advantages
of this approach are obvious -- one can aim at "the best" solution. The
inconveniencies are no less obvious -- difficulties for interacting with
existing systems, going overboard in the specification, etc.

Back in 1985, X.400 looked as a very promising solution, in particular for the
provision of multimedia mail. It did not deliver. The 1988 revision of the
standard managed to be incompatible with the "growing X.400 user base" and
severely disrupted this user base -- most ADMD, today, still use the 1984
version. In comparison, multimedia facilities and name server accesses were
incorporated in the TCP-IP mail service in a very stream-lined fashion (MIME,
DNS, PEM). If you look at the situation as of today, the advantages of X.400
are merely political -- that is, for those organizations that regard strict
obedience to ISO as a political advantage.

Christian Huitema