<s>
POPFile	B-Language
is	O
a	O
free	B-Application
,	O
open-source	B-Application
,	O
cross-platform	O
mail	O
filter	O
originally	O
written	O
in	O
Perl	B-Language
by	O
John	O
Graham-Cumming	O
and	O
maintained	O
by	O
a	O
team	O
of	O
volunteers	O
.	O
</s>
<s>
It	O
uses	O
a	O
naive	B-General_Concept
Bayes	I-General_Concept
classifier	I-General_Concept
to	O
filter	O
mail	O
.	O
</s>
<s>
In	O
the	O
most	O
popular	O
mode	O
,	O
it	O
sets	O
itself	O
up	O
as	O
a	O
proxy	B-Application
between	O
the	O
email	O
client	O
and	O
the	O
POP3	B-Protocol
server	O
.	O
</s>
<s>
As	O
mail	O
is	O
downloaded	O
via	O
POP3	B-Protocol
,	O
the	O
filter	O
identifies	O
and	O
classifies	O
mail	O
and	O
makes	O
a	O
user	O
defined	O
modification	O
to	O
the	O
subject	O
line	O
,	O
appending	O
the	O
name	O
of	O
the	O
appropriate	O
bucket	O
.	O
</s>
<s>
An	O
HTML	B-Language
based	O
interface	O
can	O
be	O
used	O
to	O
instruct	O
POPFile	B-Language
,	O
allowing	O
users	O
to	O
correct	O
errors	O
in	O
classifications	O
and	O
thus	O
train	O
the	O
system	O
to	O
be	O
sensitive	O
to	O
the	O
user	O
's	O
specific	O
requirements	O
.	O
</s>
<s>
In	O
another	O
possible	O
mode	O
,	O
POPFile	B-Language
can	O
work	O
as	O
an	O
IMAP	B-Protocol
client	O
that	O
monitors	O
an	O
IMAP	B-Protocol
server	I-Protocol
for	O
incoming	O
mail	O
and	O
also	O
for	O
messages	O
moved	O
by	O
the	O
user	O
.	O
</s>
<s>
To	O
train	O
POPFile	B-Language
in	O
this	O
mode	O
,	O
the	O
user	O
only	O
needs	O
to	O
move	O
the	O
message	O
to	O
the	O
correct	O
folder	O
,	O
i.e.	O
</s>
<s>
to	O
the	O
folder	O
where	O
POPFile	B-Language
should	O
have	O
moved	O
the	O
message	O
.	O
</s>
