<s>
Apache	B-Language
SpamAssassin	I-Language
is	O
a	O
computer	B-Application
program	I-Application
used	O
for	O
e-mail	O
spam	O
filtering	O
.	O
</s>
<s>
It	O
uses	O
a	O
variety	O
of	O
spam-detection	O
techniques	O
,	O
including	O
DNS	B-Protocol
and	O
fuzzy	O
checksum	O
techniques	O
,	O
Bayesian	B-Application
filtering	I-Application
,	O
external	O
programs	O
,	O
blacklists	O
and	O
online	O
databases	O
.	O
</s>
<s>
It	O
is	O
released	O
under	O
the	O
Apache	B-Application
License	I-Application
2.0	I-Application
and	O
is	O
a	O
part	O
of	O
the	O
Apache	O
Foundation	O
since	O
2004	O
.	O
</s>
<s>
The	O
program	O
can	O
be	O
integrated	O
with	O
the	O
mail	B-Protocol
server	I-Protocol
to	O
automatically	O
filter	O
all	O
mail	O
for	O
a	O
site	O
.	O
</s>
<s>
It	O
can	O
also	O
be	O
run	O
by	O
individual	O
users	O
on	O
their	O
own	O
mailbox	O
and	O
integrates	O
with	O
several	O
mail	B-Protocol
programs	I-Protocol
.	O
</s>
<s>
Apache	B-Language
SpamAssassin	I-Language
is	O
highly	O
configurable	O
;	O
if	O
used	O
as	O
a	O
system-wide	O
filter	O
it	O
can	O
still	O
be	O
configured	O
to	O
support	O
per-user	O
preferences	O
.	O
</s>
<s>
Apache	B-Language
SpamAssassin	I-Language
was	O
created	O
by	O
Justin	O
Mason	O
,	O
who	O
had	O
maintained	O
a	O
number	O
of	O
patches	O
against	O
an	O
earlier	O
program	O
named	O
filter.plx	O
by	O
Mark	O
Jeftovic	O
,	O
which	O
in	O
turn	O
was	O
begun	O
in	O
August	O
1997	O
.	O
</s>
<s>
Mason	O
rewrote	O
all	O
of	O
Jeftovic	O
's	O
code	O
from	O
scratch	O
and	O
uploaded	O
the	O
resulting	O
codebase	O
to	O
SourceForge	B-Application
on	O
April	O
20	O
,	O
2001	O
.	O
</s>
<s>
In	O
Summer	O
2004	O
the	O
project	O
became	O
an	O
Apache	O
Software	O
Foundation	O
project	O
and	O
later	O
officially	O
renamed	O
to	O
Apache	B-Language
SpamAssassin	I-Language
.	O
</s>
<s>
The	O
SpamAssassin	B-Language
3.4.2	O
release	O
in	O
September	O
2019	O
was	O
the	O
first	O
in	O
over	O
three	O
years	O
,	O
but	O
the	O
developers	O
say	O
that	O
"	O
The	O
project	O
has	O
picked	O
up	O
a	O
new	O
set	O
of	O
developers	O
and	O
is	O
moving	O
forward	O
again	O
.	O
</s>
<s>
In	O
December	O
2019	O
,	O
version	O
3.4.3	O
of	O
SpamAssassin	B-Language
was	O
released	O
.	O
</s>
<s>
In	O
April	O
,	O
2021	O
,	O
version	O
3.4.6	O
of	O
SpamAssassin	B-Language
was	O
released	O
.	O
</s>
<s>
Apache	B-Language
SpamAssassin	I-Language
is	O
a	O
Perl-based	O
application	O
(	O
in	O
CPAN	B-Language
)	O
which	O
is	O
usually	O
used	O
to	O
filter	O
all	O
incoming	O
mail	O
for	O
one	O
or	O
several	O
users	O
.	O
</s>
<s>
It	O
can	O
be	O
run	O
as	O
a	O
standalone	B-Operating_System
application	I-Operating_System
or	O
as	O
a	O
subprogram	O
of	O
another	O
application	O
(	O
such	O
as	O
a	O
Milter	B-Application
,	O
SA-Exim	O
,	O
Exiscan	O
,	O
MailScanner	B-Protocol
,	O
MIMEDefang	B-Protocol
,	O
Amavis	B-Language
)	O
or	O
as	O
a	O
client	B-Protocol
(	O
)	O
that	O
communicates	O
with	O
a	O
daemon	B-Operating_System
(	O
)	O
.	O
</s>
<s>
The	O
client/server	O
or	O
embedded	O
mode	O
of	O
operation	O
has	O
performance	O
benefits	O
,	O
but	O
under	O
certain	O
circumstances	O
may	O
introduce	O
additional	O
security	O
risks	O
.	O
</s>
<s>
Typically	O
either	O
variant	O
of	O
the	O
application	O
is	O
set	O
up	O
in	O
a	O
generic	O
mail	O
filter	O
program	O
,	O
or	O
it	O
is	O
called	O
directly	O
from	O
a	O
mail	B-Protocol
user	I-Protocol
agent	I-Protocol
that	O
supports	O
this	O
,	O
whenever	O
new	O
mail	O
arrives	O
.	O
</s>
<s>
Mail	O
filter	O
programs	O
such	O
as	O
procmail	B-Protocol
can	O
be	O
made	O
to	O
pipe	O
all	O
incoming	O
mail	O
through	O
Apache	B-Language
SpamAssassin	I-Language
with	O
an	O
adjustment	O
to	O
a	O
user	O
's	O
file	O
.	O
</s>
<s>
Apache	B-Language
SpamAssassin	I-Language
comes	O
with	O
a	O
large	O
set	O
of	O
rules	O
which	O
are	O
applied	O
to	O
determine	O
whether	O
an	O
email	O
is	O
spam	O
or	O
not	O
.	O
</s>
<s>
Most	O
rules	O
are	O
based	O
on	O
regular	B-Language
expressions	I-Language
that	O
are	O
matched	O
against	O
the	O
body	O
or	O
header	O
fields	O
of	O
the	O
message	O
,	O
but	O
Apache	B-Language
SpamAssassin	I-Language
also	O
employs	O
a	O
number	O
of	O
other	O
spam-fighting	O
techniques	O
.	O
</s>
<s>
The	O
rules	O
are	O
called	O
"	O
tests	O
"	O
in	O
the	O
SpamAssassin	B-Language
documentation	O
.	O
</s>
<s>
A	O
message	O
is	O
matched	O
against	O
all	O
tests	O
and	O
Apache	B-Language
SpamAssassin	I-Language
combines	O
the	O
results	O
into	O
a	O
global	O
score	O
which	O
is	O
assigned	O
to	O
the	O
message	O
.	O
</s>
<s>
Apache	B-Language
SpamAssassin	I-Language
has	O
an	O
internal	O
(	O
configurable	O
)	O
score	O
threshold	O
to	O
classify	O
a	O
message	O
as	O
spam	O
.	O
</s>
<s>
If	O
Apache	B-Language
SpamAssassin	I-Language
considers	O
a	O
message	O
to	O
be	O
spam	O
,	O
it	O
can	O
be	O
further	O
rewritten	O
.	O
</s>
<s>
In	O
the	O
default	O
configuration	O
,	O
the	O
content	O
of	O
the	O
mail	O
is	O
appended	O
as	O
a	O
MIME	B-Protocol
attachment	I-Protocol
,	O
with	O
a	O
brief	O
excerpt	O
in	O
the	O
message	O
body	O
,	O
and	O
a	O
description	O
of	O
the	O
tests	O
which	O
resulted	O
in	O
the	O
mail	O
being	O
classified	O
as	O
spam	O
.	O
</s>
<s>
Apache	B-Language
SpamAssassin	I-Language
allows	O
for	O
a	O
per-user	O
configuration	O
of	O
its	O
behavior	O
,	O
even	O
if	O
installed	O
as	O
system-wide	O
service	O
;	O
the	O
configuration	O
can	O
be	O
read	O
from	O
a	O
file	O
or	O
a	O
database	O
.	O
</s>
<s>
The	O
user	O
can	O
also	O
define	O
a	O
list	O
of	O
languages	O
which	O
they	O
want	O
to	O
receive	O
mail	O
in	O
,	O
and	O
Apache	B-Language
SpamAssassin	I-Language
then	O
assigns	O
a	O
higher	O
score	O
to	O
all	O
mails	O
that	O
appear	O
to	O
be	O
written	O
in	O
another	O
language	O
.	O
</s>
<s>
Apache	B-Language
SpamAssassin	I-Language
is	O
based	O
on	O
heuristics	O
(	O
pattern	O
recognition	O
)	O
,	O
and	O
such	O
software	O
exhibits	O
false	O
positives	O
and	O
false	O
negatives	O
.	O
</s>
<s>
Apache	B-Language
SpamAssassin	I-Language
also	O
supports	O
:	O
</s>
<s>
More	O
methods	O
can	O
be	O
added	O
reasonably	O
easily	O
by	O
writing	O
a	O
Perl	B-Language
plug-in	O
for	O
Apache	B-Language
SpamAssassin	I-Language
.	O
</s>
<s>
Apache	B-Language
SpamAssassin	I-Language
reinforces	O
its	O
rules	O
through	O
Bayesian	B-Application
filtering	I-Application
where	O
a	O
user	O
or	O
administrator	O
"	O
feeds	O
"	O
examples	O
of	O
good	O
(	O
ham	O
)	O
and	O
bad	O
(	O
spam	O
)	O
into	O
the	O
filter	O
in	O
order	O
to	O
learn	O
the	O
difference	O
between	O
the	O
two	O
.	O
</s>
<s>
For	O
this	O
purpose	O
,	O
Apache	B-Language
SpamAssassin	I-Language
provides	O
the	O
command-line	O
tool	O
,	O
which	O
can	O
be	O
instructed	O
to	O
learn	O
a	O
single	O
mail	O
or	O
an	O
entire	O
mailbox	O
as	O
either	O
ham	O
or	O
spam	O
.	O
</s>
<s>
Alternatively	O
,	O
if	O
the	O
mail	B-Protocol
user	I-Protocol
agent	I-Protocol
supports	O
it	O
,	O
can	O
be	O
called	O
for	O
individual	O
emails	O
.	O
</s>
<s>
Regardless	O
of	O
the	O
method	O
used	O
to	O
perform	O
the	O
learning	O
,	O
SpamAssassin	B-Language
's	O
Bayesian	O
test	O
will	O
help	O
score	O
future	O
e-mails	O
based	O
on	O
this	O
learning	O
to	O
improve	O
the	O
accuracy	O
.	O
</s>
<s>
Apache	B-Language
SpamAssassin	I-Language
is	O
free/open	O
source	O
software	O
,	O
licensed	O
under	O
the	O
Apache	B-Application
License	I-Application
2.0	I-Application
.	O
</s>
<s>
Versions	O
prior	O
to	O
3.0	O
are	O
dual-licensed	O
under	O
the	O
Artistic	B-License
License	I-License
and	O
the	O
GNU	B-License
General	I-License
Public	I-License
License	I-License
.	O
</s>
<s>
sa-compile	B-Language
is	O
a	O
utility	O
distributed	O
with	O
Apache	B-Language
SpamAssassin	I-Language
that	O
compiles	O
a	O
SpamAssassin	B-Language
ruleset	O
into	O
a	O
deterministic	B-General_Concept
finite	I-General_Concept
automaton	I-General_Concept
that	O
allows	O
Apache	B-Language
SpamAssassin	I-Language
to	O
use	O
processor	O
power	O
more	O
efficiently	O
.	O
</s>
<s>
Apache	B-Language
SpamAssassin	I-Language
is	O
designed	O
to	O
trigger	O
on	O
the	O
GTUBE	O
,	O
a	O
68-byte	O
string	O
similar	O
to	O
the	O
antivirus	O
EICAR	O
test	O
file	O
.	O
</s>
<s>
If	O
this	O
string	O
is	O
inserted	O
in	O
an	O
RFC	O
5322	O
formatted	O
message	O
and	O
passed	O
through	O
the	O
Apache	B-Language
SpamAssassin	I-Language
engine	O
,	O
Apache	B-Language
SpamAssassin	I-Language
will	O
trigger	O
with	O
a	O
weight	O
of	O
1000	O
.	O
</s>
