<s>
qmail	B-Application
is	O
a	O
mail	B-Protocol
transfer	I-Protocol
agent	I-Protocol
(	O
MTA	B-Protocol
)	O
that	O
runs	O
on	O
Unix	B-Application
.	O
</s>
<s>
It	O
was	O
written	O
,	O
starting	O
December	O
1995	O
,	O
by	O
Daniel	O
J	O
.	O
Bernstein	O
as	O
a	O
more	O
secure	O
replacement	O
for	O
the	O
popular	O
Sendmail	B-Application
program	O
.	O
</s>
<s>
Originally	O
license-free	B-License
software	I-License
,	O
qmail	B-Application
's	O
source	O
code	O
was	O
later	O
dedicated	O
in	O
the	O
public	O
domain	O
by	O
the	O
author	O
.	O
</s>
<s>
When	O
first	O
published	O
,	O
qmail	B-Application
was	O
the	O
first	O
security-aware	O
mail	B-Protocol
transport	I-Protocol
agent	I-Protocol
;	O
since	O
then	O
,	O
other	O
security-aware	O
MTAs	O
have	O
been	O
published	O
.	O
</s>
<s>
The	O
most	O
popular	O
predecessor	O
to	O
qmail	B-Application
,	O
Sendmail	B-Application
,	O
was	O
not	O
designed	O
with	O
security	O
as	O
a	O
goal	O
,	O
and	O
as	O
a	O
result	O
has	O
been	O
a	O
perennial	O
target	O
for	O
attackers	O
.	O
</s>
<s>
In	O
contrast	O
to	O
sendmail	B-Application
,	O
qmail	B-Application
has	O
a	O
modular	O
architecture	O
composed	O
of	O
mutually	O
untrusting	O
components	O
;	O
for	O
instance	O
,	O
the	O
SMTP	B-Protocol
listener	O
component	O
of	O
qmail	B-Application
runs	O
with	O
different	O
credentials	B-Application
from	O
the	O
queue	O
manager	O
or	O
the	O
SMTP	B-Protocol
sender	O
.	O
</s>
<s>
qmail	B-Application
was	O
also	O
implemented	O
with	O
a	O
security-aware	O
replacement	O
to	O
the	B-Language
C	I-Language
standard	I-Language
library	I-Language
,	O
and	O
as	O
a	O
result	O
has	O
not	O
been	O
vulnerable	O
to	O
stack	B-General_Concept
and	O
heap	B-General_Concept
overflows	I-General_Concept
,	O
format	O
string	O
attacks	O
,	O
or	O
temporary	O
file	O
race	B-Operating_System
conditions	I-Operating_System
.	O
</s>
<s>
When	O
it	O
was	O
released	O
,	O
qmail	B-Application
was	O
significantly	O
faster	O
than	O
Sendmail	B-Application
,	O
particularly	O
for	O
bulk	O
mail	O
tasks	O
such	O
as	O
mailing	O
list	O
servers	O
.	O
</s>
<s>
qmail	B-Application
was	O
originally	O
designed	O
as	O
a	O
way	O
for	O
managing	O
large	O
mailing	O
lists	O
.	O
</s>
<s>
At	O
the	O
time	O
of	O
qmail	B-Application
's	O
introduction	O
,	O
Sendmail	B-Application
configuration	O
was	O
notoriously	O
complex	O
,	O
while	O
qmail	B-Application
was	O
simple	O
to	O
configure	O
and	O
deploy	O
.	O
</s>
<s>
qmail	B-Application
encourages	O
the	O
use	O
of	O
several	O
innovations	O
in	O
mail	O
(	O
some	O
originated	O
by	O
Bernstein	O
,	O
others	O
not	O
)	O
:	O
</s>
<s>
Bernstein	O
invented	O
the	O
Maildir	O
format	O
for	O
qmail	B-Application
,	O
which	O
splits	O
individual	O
email	O
messages	O
into	O
separate	O
files	O
.	O
</s>
<s>
Unlike	O
the	O
de	O
facto	O
standard	O
mbox	B-Application
format	O
,	O
which	O
stored	O
all	O
messages	O
in	O
a	O
single	O
file	O
,	O
Maildir	O
avoids	O
many	O
locking	O
and	O
concurrency	O
problems	O
,	O
and	O
can	O
safely	O
be	O
provisioned	O
over	O
NFS	B-Protocol
.	O
</s>
<s>
qmail	B-Application
also	O
delivers	O
to	O
mbox	B-Application
mailboxes	O
.	O
</s>
<s>
qmail	B-Application
introduced	O
the	O
concept	O
of	O
user-controlled	O
wildcards	O
.	O
</s>
<s>
Out	O
of	O
the	O
box	O
,	O
mail	O
addressed	O
to	O
"	O
user-wildcard	O
"	O
on	O
qmail	B-Application
hosts	O
is	O
delivered	O
to	O
separate	O
mailboxes	O
,	O
allowing	O
users	O
to	O
publish	O
multiple	O
mail	O
addresses	O
for	O
mailing	O
lists	O
and	O
spam	O
management	O
.	O
</s>
<s>
qmail	B-Application
also	O
introduces	O
the	O
Quick	B-Protocol
Mail	I-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
QMTP	B-Protocol
)	O
and	O
Quick	B-Protocol
Mail	I-Protocol
Queuing	I-Protocol
Protocol	I-Protocol
(	O
QMQP	B-Protocol
)	O
protocols	O
.	O
</s>
<s>
qmail	B-Application
is	O
nearly	O
a	O
completely	O
modular	O
system	O
in	O
which	O
each	O
major	O
function	O
is	O
separated	O
from	O
the	O
other	O
major	O
functions	O
.	O
</s>
<s>
It	O
is	O
easy	O
to	O
replace	O
any	O
part	O
of	O
the	O
qmail	B-Application
system	O
with	O
a	O
different	O
module	O
as	O
long	O
as	O
the	O
new	O
module	O
retains	O
the	O
same	O
interface	O
as	O
the	O
original	O
.	O
</s>
<s>
In	O
2005	O
,	O
security	O
researcher	O
Georgi	O
Guninski	O
found	O
an	O
integer	B-Error_Name
overflow	I-Error_Name
in	O
qmail	B-Application
.	O
</s>
<s>
On	O
64-bit	O
platforms	O
,	O
in	O
default	O
configurations	O
with	O
sufficient	O
virtual	O
memory	O
,	O
the	O
delivery	O
of	O
huge	O
amounts	O
of	O
data	O
to	O
certain	O
qmail	B-Application
components	O
may	O
allow	O
remote	O
code	O
execution	O
.	O
</s>
<s>
Bernstein	O
disputes	O
that	O
this	O
is	O
a	O
practical	O
attack	O
,	O
arguing	O
that	O
no	O
real-world	O
deployment	O
of	O
qmail	B-Application
would	O
be	O
susceptible	O
.	O
</s>
<s>
Configuration	O
of	O
resource	O
limits	O
for	O
qmail	B-Application
components	O
mitigates	O
the	O
vulnerability	O
.	O
</s>
<s>
At	O
a	O
slide	O
presentation	O
the	O
following	O
day	O
,	O
Bernstein	O
stated	O
that	O
there	O
were	O
4	O
"	O
known	O
bugs	O
"	O
in	O
the	O
ten-year-old	O
qmail-1.03	O
,	O
none	O
of	O
which	O
were	O
"	O
security	O
holes	O
"	O
.	O
</s>
<s>
The	O
core	O
qmail	B-Application
package	O
has	O
not	O
been	O
updated	O
for	O
many	O
years	O
.	O
</s>
<s>
qmail	B-Application
was	O
not	O
designed	O
to	O
replace	O
Sendmail	B-Application
,	O
and	O
does	O
not	O
behave	O
exactly	O
as	O
Sendmail	B-Application
did	O
in	O
all	O
situations	O
.	O
</s>
<s>
For	O
instance	O
,	O
qmail	B-Application
's	O
approach	O
to	O
bounce	O
messages	O
(	O
a	O
format	O
called	O
QSBMF	O
)	O
differs	O
from	O
the	O
standard	O
format	O
of	O
delivery	O
status	O
notifications	O
specified	O
by	O
the	O
IETF	O
in	O
RFC	O
1894	O
,	O
meanwhile	O
advanced	O
to	O
draft	O
standard	O
as	O
RFC	O
3464	O
,	O
and	O
recommended	O
in	O
the	O
SMTP	B-Protocol
specification	O
.	O
</s>
<s>
Furthermore	O
,	O
some	O
qmail	B-Application
features	O
have	O
been	O
criticized	O
for	O
introducing	O
mail	O
forwarding	O
complications	O
;	O
for	O
instance	O
,	O
qmail	B-Application
's	O
"	O
wildcard	O
"	O
delivery	O
mechanism	O
and	O
security	O
design	O
prevents	O
it	O
from	O
rejecting	O
messages	O
from	O
forged	O
or	O
nonexistent	O
senders	O
during	O
SMTP	B-Protocol
transactions	O
.	O
</s>
<s>
In	O
the	O
past	O
,	O
these	O
differences	O
may	O
have	O
made	O
qmail	B-Application
behave	O
differently	O
when	O
abused	O
as	O
a	O
spam	O
relay	O
,	O
though	O
modern	O
spam	O
delivery	O
techniques	O
are	O
less	O
influenced	O
by	O
bounce	O
behavior	O
.	O
</s>
<s>
qmail	B-Application
was	O
released	O
to	O
the	O
public	O
domain	O
in	O
November	O
2007	O
.	O
</s>
<s>
Until	O
November	O
2007	O
,	O
qmail	B-Application
was	O
license-free	B-License
software	I-License
,	O
with	O
permission	O
granted	O
for	O
distribution	O
in	O
source	O
form	O
or	O
in	O
pre-compiled	O
form	O
(	O
a	O
"	O
var-qmail	O
package	O
"	O
)	O
only	O
if	O
certain	O
restrictions	O
(	O
primarily	O
involving	O
compatibility	O
)	O
were	O
met	O
.	O
</s>
<s>
This	O
unusual	O
licensing	O
arrangement	O
made	O
qmail	B-Application
non-free	O
according	O
to	O
some	O
guidelines	O
(	O
such	O
as	O
the	O
DFSG	B-License
)	O
,	O
and	O
was	O
a	O
cause	O
of	O
controversy	O
.	O
</s>
<s>
qmail	B-Application
is	O
the	O
only	O
broadly	O
deployed	O
public	B-Application
domain	I-Application
software	I-Application
message	B-Protocol
transfer	I-Protocol
agent	I-Protocol
(	O
MTA	B-Protocol
)	O
.	O
</s>
