<s>
X.400	B-Protocol
is	O
a	O
suite	O
of	O
ITU-T	O
recommendations	O
that	O
define	O
the	O
ITU-T	O
Message	B-Application
Handling	I-Application
System	I-Application
(	O
MHS	O
)	O
.	O
</s>
<s>
At	O
one	O
time	O
,	O
the	O
designers	O
of	O
X.400	B-Protocol
were	O
expecting	O
it	O
to	O
be	O
the	O
predominant	O
form	O
of	O
email	O
,	O
but	O
this	O
role	O
has	O
been	O
taken	O
by	O
the	O
SMTP-based	O
Internet	O
e-mail	O
.	O
</s>
<s>
Despite	O
this	O
,	O
it	O
has	O
been	O
widely	O
used	O
within	O
organizations	O
and	O
was	O
a	O
core	O
part	O
of	O
Microsoft	B-Operating_System
Exchange	I-Operating_System
Server	I-Operating_System
until	O
2006	O
;	O
variants	O
continue	O
to	O
be	O
important	O
in	O
military	O
and	O
aviation	O
contexts	O
.	O
</s>
<s>
The	O
first	O
X.400	B-Protocol
Recommendations	O
were	O
published	O
in	O
1984	O
(	O
Red	O
Book	O
)	O
,	O
and	O
a	O
substantially	O
revised	O
version	O
was	O
published	O
in	O
1988	O
(	O
Blue	O
Book	O
)	O
.	O
</s>
<s>
Although	O
X.400	B-Protocol
was	O
originally	O
designed	O
to	O
run	O
over	O
the	O
OSI	O
transport	O
service	O
,	O
an	O
adaptation	O
to	O
allow	O
operation	O
over	O
TCP/IP	B-Protocol
,	O
RFC	O
1006	O
,	O
has	O
become	O
the	O
most	O
popular	O
way	O
to	O
run	O
X.400	B-Protocol
.	O
</s>
<s>
Developed	O
in	O
cooperation	O
with	O
ISO/IEC	O
,	O
the	O
X.400-series	O
recommendations	O
specify	O
OSI	O
standard	O
protocols	O
for	O
exchanging	O
and	O
addressing	O
electronic	O
messages	O
.	O
</s>
<s>
The	O
companion	O
F.400-series	O
of	O
recommendations	O
define	O
Message	O
Handling	O
Services	O
built	O
on	O
Message	B-Application
Handling	I-Application
Systems	I-Application
(	O
MHS	O
)	O
,	O
as	O
well	O
as	O
access	O
to	O
and	O
from	O
the	O
MHS	O
for	O
public	O
services	O
.	O
</s>
<s>
In	O
the	O
late	O
1990s	O
,	O
the	O
ITU-T	O
consolidated	O
recommendations	O
F.400	O
and	O
X.400	B-Protocol
and	O
published	O
the	O
ITU-T	O
F.400/X.400	O
(	O
06/1999	O
)	O
recommendation	O
"	O
Message	B-Application
handling	I-Application
system	I-Application
and	O
service	O
overview	O
"	O
.	O
</s>
<s>
The	O
X.400-series	O
recommendations	O
define	O
the	O
technical	O
aspects	O
of	O
the	O
MHS	O
:	O
ITU-T	O
Rec	O
.	O
</s>
<s>
X.411	O
|	O
(	O
ISO/IEC	O
10021-4	O
)	O
defines	O
the	O
Message	O
Transfer	O
Service	O
(	O
MTS	O
)	O
and	O
its	O
functional	O
component	O
the	O
Message	B-Protocol
Transfer	I-Protocol
Agent	I-Protocol
(	O
MTA	B-Protocol
)	O
,	O
and	O
ITU-T	O
Rec	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
P1	O
protocol	O
is	O
used	O
explicitly	O
for	O
communication	O
among	O
MTAs	O
,	O
P3	O
between	O
the	O
user	B-Protocol
agent	I-Protocol
and	O
an	O
MTA	B-Protocol
,	O
and	O
P7	O
between	O
the	O
user	B-Protocol
agent	I-Protocol
and	O
message	O
store	O
.	O
</s>
<s>
X.400	B-Protocol
message	O
content	O
standards	O
are	O
defined	O
for	O
communication	O
between	O
user	O
agents	O
.	O
</s>
<s>
Exchange	B-Operating_System
Server	I-Operating_System
2007	O
does	O
not	O
use	O
the	O
MTA	B-Protocol
object	O
,	O
and	O
the	O
X.400	B-Protocol
connector	O
(	O
which	O
must	O
use	O
the	O
MTA	B-Protocol
)	O
is	O
gone	O
in	O
Exchange	B-Operating_System
Server	I-Operating_System
2007	O
.	O
</s>
<s>
There	O
are	O
no	O
longer	O
any	O
X.400	B-Protocol
default	O
proxy	O
e-mail	O
addresses	O
in	O
Exchange	B-Operating_System
Server	I-Operating_System
2007	O
.	O
</s>
<s>
Important	O
features	O
of	O
X.400	B-Protocol
include	O
structured	O
addressing	O
,	O
ASN.1	B-Protocol
binary	B-Algorithm
code	I-Algorithm
enabling	O
multimedia	O
content	O
(	O
predating	O
and	O
more	O
efficient	O
than	O
MIME	B-Protocol
)	O
,	O
and	O
integrated	O
security	O
capabilities	O
.	O
</s>
<s>
As	O
X.400	B-Protocol
inter-domain	O
relay	O
services	O
were	O
assumed	O
by	O
ITU	O
to	O
be	O
run	O
by	O
PTTs	O
,	O
X.400	B-Protocol
incorporated	O
fields	O
for	O
the	O
automated	O
transfer	O
of	O
messages	O
between	O
X.400	B-Protocol
and	O
other	O
PTT	O
services	O
,	O
such	O
as	O
Telex	O
,	O
facsimile	O
and	O
physical	O
postal	O
mail	O
.	O
</s>
<s>
X.404	O
|	O
ISO/IEC	O
10021-11	O
)	O
,	O
but	O
the	O
initial	O
misconception	O
that	O
X.400	B-Protocol
required	O
PTT	O
relay	O
services	O
,	O
coupled	O
with	O
PTT	O
volume-based	O
charges	O
for	O
these	O
,	O
were	O
factors	O
that	O
inhibited	O
the	O
widespread	O
uptake	O
of	O
X.400	B-Protocol
.	O
</s>
<s>
In	O
turn	O
,	O
major	O
computer	O
vendors	O
committed	O
to	O
producing	O
OSI-compliant	O
products	O
,	O
including	O
X.400	B-Protocol
.	O
</s>
<s>
Microsoft	B-Operating_System
's	I-Operating_System
Exchange	I-Operating_System
Server	I-Operating_System
was	O
developed	O
in	O
this	O
time	O
period	O
,	O
and	O
internally	O
based	O
on	O
X.400/X.500	O
-	O
with	O
the	O
initial	O
release	O
"	O
equally	O
happy	O
to	O
dispatch	O
messages	O
via	O
Messaging	O
API	O
(	O
MAPI	O
)	O
,	O
X.400	B-Protocol
,	O
or	O
Simple	B-Protocol
Mail	I-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
(	O
SMTP	B-Protocol
)	O
"	O
.	O
</s>
<s>
In	O
North	O
America	O
,	O
many	O
large	O
defense	O
contractors	O
and	O
universities	O
had	O
also	O
already	O
committed	O
to	O
the	O
Internet	O
and	O
TCP/IP	B-Protocol
standards	O
,	O
including	O
SMTP	B-Protocol
for	O
email	O
.	O
</s>
<s>
There	O
,	O
X.400	B-Protocol
is	O
still	O
used	O
in	O
some	O
applications	O
,	O
such	O
as	O
the	O
military	O
,	O
intelligence	O
services	O
and	O
aviation	O
,	O
mainly	O
because	O
the	O
X.400	B-Protocol
functions	O
for	O
integrity	O
and	O
security	O
were	O
developed	O
and	O
deployed	O
much	O
earlier	O
than	O
their	O
SMTP	B-Protocol
counterparts	O
(	O
S/MIME	B-Protocol
,	O
PGP	B-Application
and	O
SMTP-TLS	O
)	O
.	O
</s>
<s>
In	O
Europe	O
,	O
South	O
America	O
,	O
and	O
Asia	O
,	O
X.400	B-Protocol
is	O
quite	O
widely	O
implemented	O
,	O
especially	O
for	O
EDI	O
services	O
.	O
</s>
<s>
X.400	B-Protocol
has	O
been	O
extended	O
for	O
use	O
in	O
military	O
applications	O
(	O
see	O
MMHS	O
)	O
and	O
aviation	O
(	O
see	O
AMHS	O
)	O
.	O
</s>
<s>
One	O
of	O
the	O
core	O
issues	O
X.400	B-Protocol
attempted	O
to	O
address	O
was	O
the	O
failure	O
of	O
an	O
email	O
to	O
be	O
properly	O
delivered	O
when	O
the	O
address	O
was	O
not	O
specified	O
correctly	O
.	O
</s>
<s>
To	O
solve	O
this	O
problem	O
,	O
the	O
X.400	B-Protocol
address	O
scheme	O
included	O
several	O
redundant	O
fields	O
that	O
could	O
be	O
used	O
to	O
help	O
deliver	O
the	O
message	O
.	O
</s>
<s>
This	O
meant	O
that	O
as	O
long	O
as	O
the	O
message	O
reached	O
the	O
service	O
provider	O
,	O
indicated	O
by	O
the	O
"	O
Administration	O
Management	O
Domain	O
"	O
(	O
ADMD	B-Protocol
)	O
portion	O
of	O
the	O
address	O
,	O
the	O
system	O
would	O
likely	O
know	O
of	O
the	O
user	O
in	O
question	O
.	O
</s>
<s>
This	O
is	O
the	O
dominant	O
model	O
today	O
,	O
where	O
companies	O
use	O
an	O
internal	O
server	O
,	O
or	O
even	O
more	O
commonly	O
,	O
use	O
a	O
provider	O
like	O
Microsoft	O
365	O
,	O
or	O
Gmail	B-Application
,	O
which	O
is	O
invisible	O
outside	O
the	O
organization	O
,	O
and	O
even	O
to	O
the	O
users	O
.	O
</s>
<s>
In	O
this	O
model	O
,	O
the	O
ADMD	B-Protocol
is	O
unknown	O
or	O
the	O
same	O
as	O
the	O
organization	O
itself	O
.	O
</s>
<s>
This	O
made	O
trivial	O
things	O
,	O
like	O
printing	O
the	O
address	O
on	O
a	O
business	O
card	O
or	O
typing	O
it	O
into	O
the	O
email	B-Protocol
client	I-Protocol
more	O
difficult	O
than	O
simpler	O
systems	O
like	O
those	O
found	O
in	O
SMTP	B-Protocol
.	O
</s>
<s>
The	O
unwieldiness	O
of	O
this	O
addressing	O
format	O
is	O
believed	O
by	O
many	O
to	O
be	O
one	O
factor	O
in	O
the	O
lack	O
of	O
success	O
of	O
X.400	B-Protocol
.	O
</s>
<s>
An	O
X.400	B-Protocol
address	O
is	O
technically	O
referred	O
to	O
as	O
an	O
Originator/Recipient	O
(	O
OR	O
)	O
address	O
.	O
</s>
<s>
1984	O
defined	O
an	O
OR	O
address	O
as	O
an	O
X.400	B-Protocol
address	O
that	O
identified	O
where	O
the	O
user	O
is	O
located	O
.	O
</s>
<s>
1988	O
defines	O
it	O
as	O
a	O
combination	O
of	O
a	O
directory	O
name	O
(	O
distinguished	B-Protocol
name	I-Protocol
)	O
and	O
an	O
X.400	B-Protocol
address	O
.	O
</s>
<s>
An	O
X.400	B-Protocol
address	O
consists	O
of	O
several	O
elements	O
,	O
including	O
:	O
</s>
<s>
In	O
the	O
1988	O
X.400	B-Protocol
Recommendations	O
,	O
four	O
forms	O
of	O
addressing	O
were	O
delineated	O
.	O
</s>
<s>
The	O
confusion	O
caused	O
by	O
the	O
X.400	B-Protocol
addressing	O
format	O
led	O
to	O
the	O
creation	O
of	O
the	O
X.500	B-Protocol
standard	O
for	O
directory	O
services	O
.	O
</s>
<s>
The	O
idea	O
was	O
to	O
create	O
a	O
hierarchical	O
and	O
standardized	O
email	O
address	O
directory	O
with	O
replication	B-Architecture
and	O
distribution	O
functionality	O
that	O
allowed	O
multiple	O
organizations	O
to	O
produce	O
a	O
single	O
public	O
directory	O
.	O
</s>
<s>
For	O
instance	O
,	O
each	O
Administration	O
Management	O
Domain	O
(	O
service	O
provider	O
)	O
could	O
optionally	O
upload	O
their	O
directory	O
to	O
a	O
shared	O
X.500	B-Protocol
server	O
,	O
and	O
then	O
allow	O
this	O
database	O
to	O
be	O
searched	O
by	O
the	O
X.400	B-Protocol
User	O
Agents	O
during	O
email	O
creation	O
,	O
thereby	O
avoiding	O
needing	O
to	O
know	O
anything	O
about	O
the	O
address	O
other	O
than	O
the	O
recipient	O
's	O
name	O
and	O
some	O
sort	O
of	O
organizational	O
name	O
like	O
a	O
company	O
.	O
</s>
<s>
Unfortunately	O
,	O
the	O
X.500	B-Protocol
protocol	O
proved	O
to	O
be	O
every	O
bit	O
as	O
complex	O
and	O
unwieldy	O
as	O
X.400	B-Protocol
,	O
and	O
this	O
led	O
to	O
the	O
creation	O
of	O
the	O
Lightweight	B-Protocol
Directory	I-Protocol
Access	I-Protocol
Protocol	I-Protocol
,	O
or	O
LDAP	B-Protocol
,	O
that	O
standardized	O
a	O
simple	O
subset	O
of	O
the	O
X.500	B-Protocol
protocols	O
suitable	O
for	O
use	O
by	O
end-user	O
software	O
searching	O
for	O
addresses	O
.	O
</s>
<s>
LDAP	B-Protocol
is	O
widely	O
used	O
in	O
directory	O
services	O
like	O
Microsoft	O
's	O
Active	B-Protocol
Directory	I-Protocol
.	O
</s>
<s>
There	O
was	O
no	O
reason	O
for	O
the	O
organizations	O
to	O
share	O
this	O
information	O
with	O
anyone	O
else	O
;	O
as	O
RFC2693	O
put	O
it	O
,	O
"	O
imagine	O
the	O
CIA	O
adding	O
its	O
directory	O
of	O
agents	O
to	O
a	O
world-wide	O
X.500	B-Protocol
pool	O
"	O
.	O
</s>
