<s>
procmail	B-Protocol
is	O
an	O
email	B-Protocol
server	I-Protocol
software	O
component	O
—	O
specifically	O
,	O
a	O
message	B-Protocol
delivery	I-Protocol
agent	I-Protocol
(	O
MDA	O
)	O
.	O
</s>
<s>
It	O
is	O
typically	O
used	O
in	O
Unix-like	B-Operating_System
mail	O
systems	O
,	O
using	O
the	O
mbox	B-Application
and	O
Maildir	O
storage	O
formats	O
.	O
</s>
<s>
procmail	B-Protocol
was	O
first	O
developed	O
in	O
1990	O
,	O
by	O
Stephen	O
R	O
.	O
van	O
den	O
Berg	O
.	O
</s>
<s>
The	O
most	O
common	O
use	O
case	O
for	O
procmail	B-Protocol
is	O
filter	O
mail	O
into	O
different	O
mailboxes	O
,	O
based	O
on	O
criteria	O
such	O
as	O
sender	O
address	O
,	O
subject	O
keywords	O
,	O
and/or	O
mailing	O
list	O
address	O
.	O
</s>
<s>
Another	O
use	O
is	O
to	O
let	O
procmail	B-Protocol
call	O
an	O
external	O
spam	O
filter	O
program	O
,	O
such	O
as	O
SpamAssassin	B-Language
.	O
</s>
<s>
The	O
procmail	B-Protocol
developers	O
have	O
built	O
a	O
mailing	O
list	O
manager	O
called	O
SmartList	O
on	O
top	O
of	O
procmail	B-Protocol
.	O
</s>
<s>
Procmail	B-Protocol
is	O
an	O
early	O
example	O
of	O
a	O
mail	O
filtering	O
tool	O
and	O
language	O
.	O
</s>
<s>
It	O
is	O
a	O
data-driven	B-Application
programming	I-Application
language	O
,	O
similar	O
to	O
earlier	O
line-oriented	O
languages	O
such	O
as	O
sed	B-Application
and	O
AWK	B-Application
.	O
</s>
<s>
procmail	B-Protocol
reads	O
mail	O
messages	O
given	O
to	O
it	O
on	O
standard	O
input	O
,	O
delivering	O
or	O
otherwise	O
dispositioning	O
each	O
one	O
.	O
</s>
<s>
procmail	B-Protocol
is	O
typically	O
not	O
invoked	O
directly	O
by	O
the	O
user	O
.	O
</s>
<s>
Rather	O
,	O
some	O
other	O
mail	O
program	O
will	O
call	O
upon	O
procmail	B-Protocol
to	O
deliver	O
a	O
message	O
according	O
to	O
the	O
user	O
's	O
wishes	O
.	O
</s>
<s>
Message	B-Protocol
transfer	I-Protocol
agents	I-Protocol
(	O
MTAs	O
)	O
,	O
such	O
as	O
Sendmail	B-Application
or	O
Postfix	B-Application
)	O
,	O
can	O
be	O
configured	O
to	O
use	O
procmail	B-Protocol
to	O
deliver	O
all	O
mail	O
.	O
</s>
<s>
A	O
mail	B-Protocol
retrieval	I-Protocol
agent	I-Protocol
such	O
as	O
fetchmail	B-Protocol
can	O
invoke	O
procmail	B-Protocol
as	O
needed	O
.	O
</s>
<s>
The	O
companion	O
tool	O
formail	B-Protocol
allows	O
procmail	B-Protocol
to	O
be	O
applied	O
to	O
mail	O
already	O
in	O
a	O
mailbox	O
.	O
</s>
<s>
procmail	B-Protocol
's	O
behavior	O
is	O
controlled	O
by	O
a	O
config	B-Protocol
file	I-Protocol
(	O
by	O
default	O
,	O
in	O
the	O
user	O
's	O
home	B-Application
directory	I-Application
)	O
containing	O
one	O
or	O
more	O
recipes	O
,	O
read	O
in	O
order	O
.	O
</s>
<s>
Conditions	O
are	O
usually	O
extended	O
regular	B-Language
expressions	I-Language
,	O
although	O
there	O
are	O
other	O
forms	O
.	O
</s>
<s>
A	O
simple	O
config	B-Protocol
file	I-Protocol
might	O
look	O
like	O
this	O
:	O
</s>
<s>
By	O
default	O
,	O
procmail	B-Protocol
will	O
"	O
bounce	O
"	O
undeliverable	O
mail	O
back	O
to	O
the	O
sender	O
,	O
or	O
discard	O
it	O
entirely	O
.	O
</s>
<s>
A	O
mistake	O
in	O
the	O
config	B-Protocol
file	I-Protocol
can	O
result	O
in	O
all	O
mail	O
being	O
rejected	O
or	O
discarded	O
.	O
</s>
<s>
The	O
configuration	B-Protocol
file	I-Protocol
format	O
is	O
sometimes	O
regarded	O
as	O
cryptic	O
.	O
</s>
<s>
procmail	B-Protocol
is	O
not	O
well-suited	O
for	O
processing	O
mail	O
involving	O
Multipurpose	B-Protocol
Internet	I-Protocol
Mail	I-Protocol
Extensions	I-Protocol
(	O
MIME	B-Protocol
)	O
.	O
</s>
<s>
It	O
cannot	O
decode	O
or	O
recognize	O
MIME-specific	O
headers	O
or	O
international	O
characters	O
.	O
</s>
<s>
For	O
approximately	O
ten	O
years	O
,	O
procmail	B-Protocol
was	O
not	O
maintained	O
,	O
and	O
multiple	O
serious	O
security	O
vulnerabilities	O
were	O
discovered	O
in	O
the	O
intervening	O
time	O
span	O
(	O
since	O
fixed	O
)	O
.	O
</s>
