<s>
Milter	B-Application
(	O
portmanteau	O
for	O
mail	O
filter	O
)	O
is	O
an	O
extension	O
to	O
the	O
widely	O
used	O
open	O
source	O
mail	B-Protocol
transfer	I-Protocol
agents	I-Protocol
(	O
MTA	O
)	O
Sendmail	B-Application
and	O
Postfix	B-Application
.	O
</s>
<s>
It	O
allows	O
administrators	O
to	O
add	O
mail	O
filters	O
for	O
filtering	O
spam	O
or	O
viruses	B-Application
in	O
the	O
mail-processing	O
chain	O
.	O
</s>
<s>
In	O
the	O
language	O
of	O
the	O
art	O
,	O
"	O
milter	B-Application
"	O
refers	O
to	O
the	O
protocol	O
and	O
API	B-Application
implementing	O
the	O
service	O
,	O
while	O
"	O
a	O
milter	B-Application
"	O
has	O
come	O
to	O
refer	O
to	O
a	O
filter	O
application	O
that	O
uses	O
milter	B-Application
to	O
provide	O
service	O
.	O
</s>
<s>
Prior	O
to	O
the	O
advent	O
of	O
milter	B-Application
,	O
an	O
email	O
filter	O
was	O
generally	O
implemented	O
as	O
a	O
program	O
to	O
which	O
an	O
MTA	O
would	O
hand	O
the	O
message	O
once	O
it	O
has	O
completely	O
arrived	O
,	O
with	O
most	O
of	O
the	O
message	O
's	O
envelope	O
information	O
removed	O
.	O
</s>
<s>
An	O
MTA	O
that	O
is	O
milter-capable	O
instead	O
notifies	O
filters	O
to	O
which	O
it	O
is	O
connected	O
about	O
each	O
phase	O
of	O
the	O
delivery	O
of	O
a	O
message	O
,	O
from	O
initial	O
client	O
connection	O
through	O
completion	O
of	O
transmission	O
.	O
</s>
<s>
At	O
each	O
phase	O
of	O
the	O
SMTP	B-Protocol
session	O
,	O
the	O
filter	O
is	O
given	O
data	O
about	O
the	O
arriving	O
message	O
and	O
then	O
has	O
an	O
opportunity	O
to	O
terminate	O
acceptance	O
of	O
the	O
message	O
early	O
when	O
appropriate	O
.	O
</s>
<s>
Moreover	O
,	O
unlike	O
the	O
former	O
model	O
,	O
a	O
milter-capable	O
MTA	O
can	O
connect	O
to	O
multiple	O
filters	O
in	O
parallel	O
that	O
serve	O
specific	O
purposes	O
such	O
as	O
anti-virus	O
,	O
anti-spam	O
,	O
message	O
authentication	O
,	O
flow	O
regulation	O
,	O
etc	O
.	O
</s>
<s>
The	O
Sendmail	B-Application
Consortium	O
includes	O
a	O
special	O
thread-based	O
library	O
in	O
the	O
sendmail	B-Application
distribution	O
that	O
provides	O
the	O
milter	B-Application
API	B-Application
.	O
</s>
<s>
Milter	B-Application
was	O
first	O
officially	O
introduced	O
to	O
Sendmail	B-Application
8.12.0	O
released	O
on	O
September	O
8	O
,	O
2001	O
.	O
</s>
<s>
Milter	B-Application
was	O
available	O
in	O
sendmail	B-Application
8.11.x	O
versions	O
as	O
an	O
FFR	O
(	O
For	O
Future	O
Release/Beta	O
code	O
)	O
.	O
</s>
<s>
Milter	B-Application
has	O
undergone	O
substantial	O
improvements	O
since	O
its	O
first	O
official	O
release	O
.	O
</s>
<s>
Milter	B-Application
support	O
was	O
added	O
to	O
Postfix	B-Application
2.3.0	O
released	O
July	O
12	O
,	O
2006	O
.	O
</s>
<s>
Many	O
,	O
but	O
not	O
all	O
,	O
milters	B-Application
can	O
be	O
used	O
with	O
Postfix	B-Application
"	O
out	O
of	O
the	O
box	O
"	O
.	O
</s>
<s>
The	O
Sendmail	B-Application
Consortium	O
provides	O
a	O
documented	O
Application	B-Application
Programming	I-Application
Interface	I-Application
that	O
can	O
be	O
used	O
to	O
create	O
milters	B-Application
for	O
special	O
needs	O
.	O
</s>
<s>
The	O
library	O
provided	O
by	O
The	O
Sendmail	B-Application
Consortium	O
is	O
pthread-based	O
.	O
</s>
<s>
However	O
,	O
documentation	O
of	O
the	O
protocol	O
used	O
for	O
communication	O
between	O
sendmail	B-Application
and	O
milter	B-Application
processes	O
is	O
not	O
provided	O
.	O
</s>
<s>
This	O
internal	O
protocol	O
is	O
subject	O
to	O
changes	O
in	O
new	O
sendmail	B-Application
versions	O
.	O
</s>
