<s>
Postfix	B-Application
is	O
a	O
free	B-Application
and	O
open-source	B-Application
mail	B-Protocol
transfer	I-Protocol
agent	I-Protocol
(	O
MTA	B-Protocol
)	O
that	O
routes	O
and	O
delivers	O
electronic	O
mail	O
.	O
</s>
<s>
It	O
is	O
released	O
under	O
the	O
IBM	B-License
Public	I-License
License	I-License
1.0	O
which	O
is	O
a	O
free	B-License
software	I-License
license	I-License
.	O
</s>
<s>
Alternatively	O
,	O
starting	O
with	O
version	O
3.2.5	O
,	O
it	O
is	O
available	O
under	O
the	O
Eclipse	B-License
Public	I-License
License	I-License
2.0	O
at	O
the	O
user	O
's	O
option	O
.	O
</s>
<s>
Originally	O
written	O
in	O
1997	O
by	O
Wietse	O
Venema	O
at	O
the	O
IBM	O
Thomas	O
J	O
.	O
Watson	O
Research	O
Center	O
in	O
New	O
York	O
,	O
and	O
first	O
released	O
in	O
December	O
1998	O
,	O
Postfix	B-Application
continues	O
to	O
be	O
actively	O
developed	O
by	O
its	O
creator	O
and	O
other	O
contributors	O
.	O
</s>
<s>
The	O
software	O
is	O
also	O
known	O
by	O
its	O
former	O
names	O
VMailer	B-Application
and	O
IBM	B-Application
Secure	I-Application
Mailer	I-Application
.	O
</s>
<s>
The	O
name	O
Postfix	B-Application
is	O
a	O
compound	O
of	O
"	O
post	O
"	O
(	O
which	O
is	O
another	O
word	O
for	O
"	O
mail	O
"	O
)	O
and	O
"	O
fix	O
"	O
(	O
which	O
is	O
for	O
other	O
software	O
that	O
inspired	O
Postfix	B-Application
development	O
)	O
.	O
</s>
<s>
In	O
a	O
study	O
published	O
on	O
June	O
1	O
,	O
2022	O
by	O
E-Soft	O
,	O
Inc.	O
,	O
approximately	O
34%	O
of	O
the	O
publicly	O
reachable	O
mail-servers	O
on	O
the	O
Internet	O
ran	O
Postfix	B-Application
,	O
making	O
it	O
the	O
second	O
most	O
popular	O
mail	B-Protocol
server	I-Protocol
behind	O
Exim	B-Protocol
.	O
</s>
<s>
Note	O
:	O
this	O
survey	O
covers	O
less	O
than	O
25%	O
of	O
the	O
4+	O
million	O
reachable	O
SMTP	B-Protocol
servers	I-Protocol
on	O
the	O
Internet	O
.	O
</s>
<s>
As	O
an	O
SMTP	B-Protocol
server	I-Protocol
,	O
Postfix	B-Application
implements	O
a	O
first	O
layer	O
of	O
defense	O
against	O
spambots	B-Protocol
and	O
malware	O
.	O
</s>
<s>
Administrators	O
can	O
combine	O
Postfix	B-Application
with	O
other	O
software	O
that	O
provides	O
spam/virus	O
filtering	O
(	O
e.g.	O
,	O
Amavisd-new	B-Language
)	O
,	O
message-store	O
access	O
(	O
e.g.	O
,	O
Dovecot	B-Language
)	O
,	O
or	O
complex	O
SMTP-level	O
access-policies	O
(	O
e.g.	O
,	O
,	O
,	O
policyd-weight	O
)	O
.	O
</s>
<s>
As	O
an	O
SMTP	B-Protocol
client	B-Protocol
,	O
Postfix	B-Application
implements	O
a	O
high-performance	O
parallelized	O
mail-delivery	O
engine	O
.	O
</s>
<s>
Postfix	B-Application
is	O
often	O
combined	O
with	O
mailing-list	O
software	O
(	O
such	O
as	O
Mailman	B-Application
)	O
.	O
</s>
<s>
Postfix	B-Application
runs	O
(	O
or	O
has	O
run	O
)	O
on	O
AIX	B-Application
,	O
BSD	B-Operating_System
,	O
HP-UX	B-Application
,	O
Linux	B-Application
,	O
macOS	B-Application
,	O
Solaris	B-Application
and	O
,	O
generally	O
speaking	O
,	O
on	O
every	O
Unix-like	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
that	O
ships	O
with	O
a	O
C	B-Language
compiler	B-Language
and	O
delivers	O
a	O
standard	O
POSIX	O
development	O
environment	O
.	O
</s>
<s>
macOS	B-Application
,	O
NetBSD	B-Device
,	O
</s>
<s>
RedHat/CentOS	O
and	O
Ubuntu	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
Postfix	B-Application
consists	O
of	O
a	O
combination	O
of	O
server	B-Application
programs	O
that	O
run	O
in	O
the	O
background	O
,	O
and	O
client	B-Protocol
programs	I-Protocol
that	O
are	O
invoked	O
by	O
user	O
programs	O
or	O
by	O
system	O
administrators	O
.	O
</s>
<s>
The	O
Postfix	B-Application
core	O
consists	O
of	O
several	O
dozen	O
server	B-Application
programs	O
that	O
run	O
in	O
the	O
background	O
,	O
each	O
handling	O
one	O
specific	O
aspect	O
of	O
email	O
delivery	O
.	O
</s>
<s>
Examples	O
are	O
the	O
SMTP	B-Protocol
server	I-Protocol
,	O
the	O
scheduler	O
,	O
the	O
address	O
rewriter	O
,	O
and	O
the	O
local	O
delivery	O
server	B-Application
.	O
</s>
<s>
For	O
damage-control	O
purposes	O
,	O
most	O
server	B-Application
programs	O
run	O
with	O
fixed	O
reduced	O
privileges	O
,	O
and	O
terminate	O
voluntarily	O
after	O
processing	O
a	O
limited	O
number	O
of	O
requests	O
.	O
</s>
<s>
To	O
conserve	O
system	O
resources	O
,	O
most	O
server	B-Application
programs	O
terminate	O
when	O
they	O
become	O
idle	O
.	O
</s>
<s>
Client	B-Protocol
programs	I-Protocol
run	O
outside	O
the	O
Postfix	B-Application
core	O
.	O
</s>
<s>
They	O
interact	O
with	O
Postfix	B-Application
server	B-Application
programs	O
through	O
mail	O
delivery	O
instructions	O
in	O
the	O
user	O
's	O
~	O
/.forward	O
file	O
,	O
and	O
through	O
small	O
"	O
gate	O
"	O
programs	O
to	O
submit	O
mail	O
or	O
to	O
request	O
queue	O
status	O
information	O
.	O
</s>
<s>
Other	O
programs	O
provide	O
administrative	O
support	O
to	O
start	O
or	O
stop	O
Postfix	B-Application
,	O
query	O
status	O
information	O
,	O
manipulate	O
the	O
queue	O
,	O
or	O
to	O
examine	O
or	O
update	O
its	O
configuration	O
files	O
.	O
</s>
<s>
Yellow	O
ellipses	O
One	O
of	O
Postfix	B-Application
 '	O
many	O
daemons	O
serving	O
exactly	O
one	O
purpose	O
.	O
</s>
<s>
This	O
split-up	O
into	O
many	O
smaller	O
pieces	O
of	O
software	O
is	O
considered	O
one	O
of	O
the	O
reasons	O
why	O
Postfix	B-Application
is	O
secure	O
and	O
stable	O
.	O
</s>
<s>
White	O
clouds	O
The	O
clouds	O
stand	O
for	O
points	O
at	O
which	O
e-mails	O
enter	O
or	O
leave	O
Postfix	B-Application
.	O
</s>
<s>
For	O
example	O
,	O
smtpd	O
receives	O
mail	O
from	O
other	O
mail	B-Protocol
servers	I-Protocol
or	O
users	O
whereas	O
smtp	B-Protocol
relays	O
mail	O
to	O
other	O
MTAs	O
.	O
</s>
<s>
The	O
Postfix	B-Application
implementation	O
uses	O
safe	O
subsets	O
of	O
the	O
C	B-Language
language	I-Language
and	O
of	O
the	O
POSIX	O
system	O
API	B-Application
.	O
</s>
<s>
These	O
subsets	O
are	O
buried	O
under	O
an	O
abstraction	O
layer	O
that	O
contains	O
about	O
50%	O
of	O
all	O
Postfix	B-Application
source	O
code	O
,	O
and	O
that	O
provides	O
the	O
foundation	O
on	O
which	O
all	O
Postfix	B-Application
programs	O
are	O
built	O
.	O
</s>
<s>
API	B-Application
.	O
</s>
<s>
This	O
abstraction	O
layer	O
does	O
not	O
affect	O
the	O
attack	O
resistance	O
of	O
non-Postfix	O
code	O
,	O
such	O
as	O
code	O
in	O
system	O
libraries	O
or	O
in	O
third-party	O
libraries	O
.	O
</s>
<s>
Conceptually	O
,	O
Postfix	B-Application
manages	O
pipelines	B-Operating_System
of	O
processes	O
that	O
pass	O
the	O
responsibility	O
for	O
message	O
delivery	O
and	O
error	O
notification	O
from	O
one	O
process	O
to	O
the	O
next	O
.	O
</s>
<s>
The	O
processes	O
in	O
a	O
pipeline	B-Operating_System
operate	O
mostly	O
without	O
centralized	O
control	O
;	O
this	O
relative	O
autonomy	O
simplifies	O
error	O
recovery	O
.	O
</s>
<s>
When	O
a	O
process	O
fails	O
before	O
completing	O
its	O
part	O
of	O
a	O
file	O
or	O
protocol	O
transaction	O
,	O
its	O
predecessor	O
in	O
the	O
pipeline	B-Operating_System
backs	O
off	O
and	O
retries	O
the	O
request	O
later	O
,	O
and	O
its	O
successor	O
in	O
the	O
pipeline	B-Operating_System
discards	O
unfinished	O
work	O
.	O
</s>
<s>
Many	O
Postfix	B-Application
daemons	O
can	O
simply	O
"	O
die	O
"	O
when	O
they	O
run	O
into	O
a	O
problem	O
;	O
they	O
are	O
automatically	O
restarted	O
when	O
the	O
next	O
service	O
request	O
arrives	O
.	O
</s>
<s>
This	O
approach	O
makes	O
Postfix	B-Application
highly	O
resilient	O
,	O
as	O
long	O
as	O
the	O
operating	O
system	O
or	O
hardware	O
do	O
n't	O
fail	O
catastrophically	O
.	O
</s>
<s>
that	O
was	O
achieved	O
with	O
the	O
mail	O
queue	O
on	O
a	O
RAM	O
disk	O
while	O
delivering	O
to	O
the	O
""	O
transport	O
(	O
with	O
a	O
dual-core	O
Opteron	B-General_Concept
system	O
in	O
2007	O
)	O
.	O
</s>
<s>
Mail	O
systems	O
such	O
as	O
Postfix	B-Application
and	O
Qmail	B-Application
achieve	O
high	O
performance	O
by	O
delivering	O
mail	O
in	O
parallel	O
sessions	O
.	O
</s>
<s>
With	O
mail	O
systems	O
such	O
as	O
Sendmail	B-Application
and	O
Exim	B-Protocol
that	O
make	O
one	O
connection	O
at	O
a	O
time	O
,	O
high	O
performance	O
can	O
be	O
achieved	O
by	O
submitting	O
limited	O
batches	O
of	O
mail	O
in	O
parallel	O
,	O
so	O
that	O
each	O
batch	O
is	O
delivered	O
by	O
a	O
different	O
process	O
.	O
</s>
<s>
Postfix	B-Application
and	O
Qmail	B-Application
require	O
parallel	O
submission	O
into	O
different	O
MTA	B-Protocol
instances	O
once	O
they	O
reach	O
their	O
intrinsic	O
performance	O
limit	O
,	O
or	O
the	O
performance	O
limits	O
of	O
the	O
hardware	O
or	O
operating	O
system	O
.	O
</s>
<s>
The	O
file	O
stores	O
site-specific	O
Postfix	B-Application
configuration	O
parameters	O
while	O
defines	O
daemon	O
processes	O
.	O
</s>
<s>
More	O
complex	O
Postfix	B-Application
implementations	O
may	O
include	O
:	O
integration	O
with	O
other	O
applications	O
such	O
as	O
SpamAssassin	B-Language
;	O
support	O
for	O
multiple	O
virtual	O
domain	O
names	O
-	O
and	O
use	O
databases	O
such	O
as	O
MySQL	B-Application
to	O
control	O
complex	O
configurations	O
.	O
</s>
<s>
VersionRelease	O
date	O
Postfix	B-Application
Source	O
CodeIndex	O
of	O
/pub/postfix-release/officialPostfix	O
AnnouncementsSignificant	O
changesMajorMinorrowspan	O
=	O
"	O
10	O
"	O
0.8.0	O
6	O
September	O
1999	O
0.8.1	O
6	O
September	O
1999	O
0.8.2	O
6	O
September	O
1999	O
0.8.3	O
21	O
September	O
1999	O
0.8.4	O
22	O
September	O
1999	O
0.8.5	O
22	O
September	O
1999	O
0.8.6	O
6	O
November	O
1999	O
0.8.7	O
12	O
November	O
1999	O
0.8.8	O
21	O
November	O
1999	O
0.8.9	O
10	O
December	O
1999	O
rowspan	O
=	O
"	O
14	O
"	O
0.9.0	O
31	O
December	O
1999	O
0.9.1	O
1	O
January	O
2000	O
0.9.2	O
3	O
January	O
2000	O
0.9.3	O
26	O
January	O
2000	O
0.9.4	O
29	O
january	O
2000	O
0.9.5	O
8	O
March	O
2000	O
0.9.6	O
29	O
March	O
2000	O
0.9.7	O
11	O
May	O
2000	O
0.9.8	O
30	O
May	O
2000	O
0.9.9	O
17	O
September	O
2000	O
0.9.10	O
29	O
October	O
2000	O
0.9.11	O
21	O
November	O
2000	O
0.9.12	O
7	O
December	O
2000	O
0.9.13	O
9	O
December	O
2000	O
rowspan	O
=	O
"	O
9	O
"	O
1.0.0	O
28	O
February	O
2001	O
1.0.1	O
28	O
March	O
2001	O
1.0.2	O
29	O
April	O
2001	O
1.0.3	O
25	O
May	O
2001	O
1.0.4	O
30	O
July	O
2001	O
1.0.5	O
16	O
September	O
2001	O
1.0.6	O
3	O
November	O
2001	O
1.0.7	O
13	O
November	O
2001	O
1.0.8	O
14	O
November	O
2001	O
rowspan	O
=	O
"	O
14	O
"	O
1.1.0	O
17	O
January	O
2002	O
1.1.1	O
21	O
January	O
2002	O
1.1.2	O
24	O
January	O
2002	O
1.1.3	O
1	O
February	O
2002	O
1.1.4	O
25	O
February	O
2002	O
1.1.5	O
10	O
March	O
2002	O
1.1.6	O
25	O
March	O
2002	O
1.1.7	O
30	O
March	O
2002	O
1.1.8	O
4	O
May	O
2002	O
1.1.9	O
12	O
May	O
2002	O
1.1.10	O
13	O
May	O
2002	O
1.1.11	O
27	O
May	O
2002	O
1.1.12	O
21	O
Nov	O
2002	O
1.1.13	O
27	O
July	O
2003	O
rowspan	O
=	O
"	O
21	O
"	O
2.0.0	O
22	O
December	O
2002	O
2.0.1	O
23	O
February	O
2003	O
2.0.2	O
23	O
February	O
2003	O
2.0.3	O
23	O
February	O
2003	O
2.0.4	O
23	O
February	O
2003	O
2.0.5	O
2	O
March	O
2003	O
2.0.6	O
4	O
March	O
2003	O
2.0.7	O
18	O
March	O
2003	O
2.0.8	O
14	O
April	O
2003	O
2.0.9	O
17	O
April	O
2003	O
2.0.10	O
20	O
May	O
2003	O
2.0.11	O
8	O
June	O
2003	O
2.0.12	O
10	O
june	O
2003	O
2.0.13	O
28	O
June	O
2003	O
2.0.14	O
11	O
August	O
2003	O
2.0.15	O
4	O
September	O
2003	O
2.0.16	O
21	O
January	O
2004	O
2.0.17	O
21	O
January	O
2004	O
2.0.18	O
24	O
January	O
2004	O
2.0.19	O
11	O
March	O
2004	O
2.0.20	O
21	O
April	O
2004	O
rowspan	O
=	O
"	O
7	O
"	O
2.1.0	O
22	O
April	O
2004	O
2.1.1	O
2	O
May	O
2004	O
2.1.2	O
14	O
June	O
2004	O
2.1.3	O
17	O
June	O
2004	O
2.1.4	O
29	O
June	O
2004	O
2.1.5	O
14	O
September	O
2004	O
2.1.6	O
8	O
May	O
2005	O
rowspan	O
=	O
"	O
13	O
"	O
2.2.0	O
09	O
March	O
2005	O
2.2.1	O
14	O
March	O
2005	O
2.2.2	O
31	O
March	O
2005	O
2.2.3	O
28	O
April	O
2005	O
2.2.4	O
20	O
June	O
2005	O
2.2.5	O
18	O
July	O
2005	O
2.2.6	O
28	O
November	O
2005	O
2.2.7	O
7	O
December	O
2005	O
2.2.8	O
2	O
January	O
2006	O
2.2.9	O
20	O
February	O
2006	O
2.2.10	O
5	O
April	O
2006	O
2.2.11	O
23	O
July	O
2006	O
2.2.12	O
20	O
October	O
2007	O
rowspan	O
=	O
"	O
20	O
"	O
2.3.0	O
10	O
july	O
2006	O
2.3.1	O
23	O
July	O
2006	O
2.3.2	O
26	O
July	O
2006	O
2.3.3	O
24	O
August	O
2006	O
2.3.4	O
31	O
October	O
2006	O
2.3.5	O
10	O
December	O
2006	O
2.3.6	O
31	O
December	O
2006	O
2.3.7	O
29	O
January	O
2007	O
2.3.8	O
1	O
March	O
2007	O
2.3.9	O
22	O
April	O
2007	O
2.3.10	O
29	O
May	O
2007	O
2.3.11	O
30	O
May	O
2007	O
2.3.12	O
30	O
July	O
2007	O
2.3.13	O
16	O
October	O
2007	O
2.3.14	O
30	O
January	O
2008	O
2.3.15	O
4	O
August	O
2008	O
2.3.16	O
3	O
January	O
2009	O
2.3.17	O
11	O
May	O
2009	O
2.3.18	O
25	O
August	O
2009	O
2.3.19	O
27	O
August	O
2009	O
rowspan	O
=	O
"	O
17	O
"	O
2.4.0	O
22	O
April	O
2007	O
2.4.1	O
29	O
may	O
2007	O
2.4.2	O
30	O
May	O
2007	O
2.4.3	O
30	O
June	O
2007	O
2.4.4	O
30	O
July	O
2007	O
2.4.5	O
30	O
July	O
2007	O
2.4.6	O
16	O
October	O
2007	O
2.4.7	O
30	O
January	O
2008	O
2.4.8	O
4	O
August	O
2008	O
2.4.9	O
30	O
August	O
2008	O
2.4.10	O
3	O
January	O
2009	O
2.4.11	O
11	O
May	O
2009	O
2.4.12	O
25	O
August	O
2009	O
2.4.13	O
27	O
August	O
2009	O
2.4.14	O
21	O
March	O
2010	O
2.4.15	O
22	O
November	O
2010	O
2.4.16	O
28	O
February	O
2011	O
rowspan	O
=	O
"	O
18	O
"	O
2.5.0	O
24	O
January	O
2008	O
2.5.1	O
15	O
February	O
2008	O
2.5.2	O
7	O
May	O
2008	O
2.5.3	O
26	O
July	O
2008	O
2.5.4	O
13	O
August	O
2008	O
2.5.5	O
30	O
August	O
2008	O
2.5.6	O
3	O
January	O
2008	O
2.5.7	O
11	O
May	O
2009	O
2.5.8	O
28	O
August	O
2009	O
Withdrawn	O
release.2.5.9	O
28	O
August	O
2009	O
2.5.10	O
21	O
March	O
2010	O
2.5.11	O
22	O
November	O
2010	O
2.5.12	O
28	O
February	O
2011	O
2.5.13	O
18	O
April	O
2011	O
2.5.14	O
6	O
July	O
2011	O
2.5.15	O
1	O
September	O
2011	O
2.5.16	O
22	O
October	O
2011	O
2.5.17	O
6	O
February	O
2012	O
EOL	O
2.5rowspan	O
=	O
"	O
20	O
"	O
2.6.0	O
11	O
May	O
2009	O
2.6.1	O
19	O
May	O
2009	O
2.6.2	O
03	O
June	O
2009	O
2.6.3	O
02	O
August	O
2009	O
Unannounced	O
release.2.6.4	O
25	O
August	O
2009	O
Withdrawn	O
release.2.6.5	O
28	O
August	O
2009	O
2.6.6	O
19	O
March	O
2010	O
2.6.7	O
08	O
June	O
2010	O
2.6.8	O
23	O
November	O
2010	O
2.6.9	O
03	O
March	O
2011	O
2.6.10	O
09	O
May	O
2011	O
2.6.11	O
07	O
July	O
2011	O
2.6.12	O
02	O
September	O
2011	O
2.6.13	O
24	O
October	O
2011	O
2.6.14	O
04	O
February	O
2012	O
2.6.15	O
24	O
April	O
2012	O
2.6.16	O
20	O
May	O
2012	O
2.6.17	O
01	O
August	O
2012	O
2.6.18	O
13	O
December	O
2012	O
2.6.19	O
03	O
February	O
2013	O
EOL	O
2.6rowspan	O
=	O
"	O
17	O
"	O
2.7.0	O
13	O
February	O
2010	O
2.7.1	O
08	O
June	O
2010	O
2.7.2	O
23	O
November	O
2010	O
2.7.3	O
03	O
March	O
2011	O
2.7.4	O
09	O
May	O
2011	O
2.7.5	O
07	O
July	O
2011	O
2.7.6	O
02	O
September	O
2011	O
2.7.7	O
24	O
October	O
2011	O
2.7.8	O
04	O
February	O
2012	O
2.7.9	O
24	O
April	O
2012	O
2.7.10	O
20	O
May	O
2012	O
2.7.11	O
01	O
August	O
2012	O
2.7.12	O
13	O
December	O
2012	O
2.7.13	O
03	O
February	O
2013	O
2.7.14	O
22	O
June	O
2013	O
2.7.15	O
05	O
September	O
2013	O
2.7.16	O
16	O
January	O
2014	O
EOL	O
2.7rowspan	O
=	O
"	O
21	O
"	O
2.8.0	O
20	O
January	O
2011	O
Introduction	O
of	O
postscreen	O
daemon	O
that	O
sits	O
before	O
the	O
main	O
postfix	B-Application
and	O
acts	O
as	O
a	O
Zombie	O
blocker	O
.	O
</s>
