<s>
A	O
hash	B-Error_Name
filter	I-Error_Name
creates	O
a	O
hash	B-Error_Name
sum	I-Error_Name
from	O
data	O
,	O
typically	O
e-mail	O
,	O
and	O
compares	O
the	O
sum	O
against	O
other	O
previously	O
defined	O
sums	O
.	O
</s>
<s>
For	O
example	O
,	O
when	O
a	O
message	O
is	O
received	O
by	O
an	O
e-mail	O
server	O
with	O
a	O
hash	B-Error_Name
filter	I-Error_Name
,	O
the	O
contents	O
of	O
the	O
e-mail	O
is	O
converted	O
into	O
a	O
hash	B-Error_Name
sum	I-Error_Name
.	O
</s>
<s>
If	O
this	O
sum	O
corresponds	O
to	O
the	O
hash	B-Error_Name
sum	I-Error_Name
of	O
another	O
e-mail	O
which	O
has	O
been	O
categorized	O
as	O
spam	O
,	O
the	O
received	O
e-mail	O
is	O
prevented	O
from	O
being	O
delivered	O
.	O
</s>
