<s>
Exim	B-Protocol
is	O
a	O
mail	B-Protocol
transfer	I-Protocol
agent	I-Protocol
(	O
MTA	O
)	O
used	O
on	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
Exim	B-Protocol
is	O
free	B-Application
software	I-Application
distributed	O
under	O
the	O
terms	O
of	O
the	O
GNU	B-License
General	I-License
Public	I-License
License	I-License
,	O
and	O
it	O
aims	O
to	O
be	O
a	O
general	O
and	O
flexible	O
mailer	O
with	O
extensive	O
facilities	O
for	O
checking	O
incoming	O
e-mail	O
.	O
</s>
<s>
Exim	B-Protocol
has	O
been	O
ported	O
to	O
most	O
Unix-like	B-Operating_System
systems	I-Operating_System
,	O
as	O
well	O
as	O
to	O
Microsoft	B-Application
Windows	I-Application
using	O
the	O
Cygwin	B-Language
emulation	O
layer	O
.	O
</s>
<s>
Exim	B-Protocol
4	O
is	O
currently	O
the	O
default	O
MTA	O
on	O
Debian	O
Linux	B-Application
systems	O
.	O
</s>
<s>
Many	O
Exim	B-Protocol
installations	O
exist	O
,	O
especially	O
within	O
Internet	O
service	O
providers	O
and	O
universities	O
in	O
the	O
United	O
Kingdom	O
.	O
</s>
<s>
Exim	B-Protocol
is	O
also	O
widely	O
used	O
with	O
the	O
GNU	B-Application
Mailman	I-Application
mailing	O
list	O
manager	O
,	O
and	O
cPanel	B-Protocol
.	O
</s>
<s>
In	O
March	O
2023	O
a	O
study	O
performed	O
by	O
E-Soft	O
,	O
Inc.	O
,	O
approximated	O
that	O
59%	O
of	O
the	O
publicly	O
reachable	O
mail-servers	O
on	O
the	O
Internet	O
ran	O
Exim	B-Protocol
.	O
</s>
<s>
The	O
first	O
version	O
of	O
Exim	B-Protocol
was	O
written	O
in	O
1995	O
by	O
Philip	O
Hazel	O
for	O
use	O
in	O
the	O
University	O
of	O
Cambridge	O
Computing	O
Service	O
’s	O
e-mail	O
systems	O
.	O
</s>
<s>
It	O
was	O
originally	O
based	O
on	O
an	O
older	O
MTA	O
,	O
Smail-3	O
,	O
but	O
it	O
has	O
since	O
diverged	O
from	O
Smail-3	O
in	O
its	O
design	O
and	O
philosophy	O
.	O
</s>
<s>
Exim	B-Protocol
,	O
like	O
Smail	B-Protocol
,	O
still	O
follows	O
the	O
Sendmail	B-Application
design	O
model	O
,	O
where	O
a	O
single	B-Application
binary	I-Application
controls	O
all	O
the	O
facilities	O
of	O
the	O
MTA	O
.	O
</s>
<s>
Exim	B-Protocol
has	O
well-defined	O
stages	O
during	O
which	O
it	O
gains	O
or	O
loses	O
privileges	O
.	O
</s>
<s>
Exim	B-Protocol
's	O
security	O
has	O
had	O
a	O
number	O
of	O
serious	O
security	O
problems	O
diagnosed	O
over	O
the	O
years	O
.	O
</s>
<s>
Since	O
the	O
redesigned	O
version	O
4	O
was	O
released	O
there	O
have	O
been	O
four	O
remote	O
code	O
execution	O
flaws	O
and	O
one	O
conceptual	O
flaw	O
concerning	O
how	O
much	O
trust	O
it	O
is	O
appropriate	O
to	O
place	O
in	O
the	O
run-time	B-Library
user	O
;	O
the	O
latter	O
was	O
fixed	O
in	O
a	O
security	O
lockdown	O
in	O
revision	O
4.73	O
,	O
one	O
of	O
the	O
very	O
rare	O
occasions	O
when	O
Exim	B-Protocol
has	O
broken	O
backwards	B-General_Concept
compatibility	I-General_Concept
with	O
working	O
configurations	O
.	O
</s>
<s>
Exim	B-Protocol
is	O
highly	O
configurable	O
and	O
therefore	O
has	O
features	O
that	O
are	O
lacking	O
in	O
other	O
MTAs	O
.	O
</s>
<s>
The	O
integration	O
of	O
a	O
framework	O
for	O
content	O
scanning	O
,	O
which	O
allowed	O
for	O
easier	O
integration	O
of	O
anti-virus	B-Application
and	O
anti-spam	O
measures	O
,	O
happened	O
in	O
the	O
4.x	O
releases	O
.	O
</s>
<s>
This	O
made	O
Exim	B-Protocol
very	O
suitable	O
for	O
enforcing	O
diverse	O
mail	O
policies	O
.	O
</s>
<s>
the	O
access	O
control	O
list	O
(	O
ACL	O
)	O
section	O
which	O
defines	O
behaviour	O
during	O
the	O
SMTP	B-Protocol
sessions	O
,	O
</s>
<s>
the	O
authenticators	O
 '	O
section	O
with	O
settings	O
for	O
SMTP	B-Protocol
AUTH	O
,	O
a	O
rule	O
per	O
auth	O
mechanism	O
.	O
</s>
<s>
There	O
are	O
two	O
main	O
schools	O
of	O
configuration	O
style	O
for	O
Exim	B-Protocol
.	O
</s>
<s>
The	O
native	O
school	O
keeps	O
the	O
Exim	B-Protocol
configuration	O
in	O
one	O
file	O
and	O
external	O
files	O
are	O
only	O
used	O
as	O
data	O
sources	O
;	O
this	O
is	O
strongly	O
influenced	O
by	O
Philip	O
Hazel	O
's	O
preferences	O
and	O
notes	O
on	O
performance	O
as	O
the	O
configuration	O
file	O
is	O
re-read	O
at	O
every	O
exec	O
,	O
which	O
happens	O
post-fork	O
for	O
receiving	O
inbound	O
connections	O
and	O
at	O
delivery	O
.	O
</s>
<s>
In	O
these	O
approaches	O
,	O
a	O
debconf	O
configuration	O
file	O
is	O
used	O
to	O
build	O
the	O
Exim	B-Protocol
configuration	O
file	O
,	O
together	O
with	O
templates	O
and	O
directories	O
with	O
configuration	O
fragments	O
.	O
</s>
<s>
When	O
the	O
supervisor	O
for	O
exim	B-Protocol
is	O
invoked	O
it	O
re-processes	O
the	O
configuration	O
files	O
producing	O
a	O
single-file	O
configuration	O
that	O
the	O
exim	B-Protocol
binary	O
uses	O
.	O
</s>
<s>
Because	O
the	O
Debian	O
approach	O
diverges	O
significantly	O
from	O
the	O
Exim	B-Protocol
one	O
it	O
is	O
common	O
to	O
find	O
a	O
lack	O
of	O
support	O
for	O
the	O
Debian	O
approach	O
on	O
the	O
regular	O
Exim	B-Protocol
mailing-lists	O
,	O
with	O
people	O
advised	O
to	O
ask	O
Debian	O
questions	O
on	O
the	O
Debian-managed	O
mailing-list	O
.	O
</s>
<s>
Exim	B-Protocol
has	O
extensive	O
and	O
exhaustive	O
documentation	O
;	O
if	O
a	O
feature	O
or	O
some	O
behaviour	O
is	O
not	O
documented	O
then	O
this	O
is	O
classed	O
as	O
a	O
bug	O
.	O
</s>
<s>
The	O
documentation	O
consists	O
of	O
The	O
Exim	B-Protocol
Specification	O
and	O
two	O
ancillary	O
files	O
:	O
the	O
experimental	O
specification	O
for	O
features	O
that	O
might	O
disappear	O
and	O
"	O
NewStuff	O
"	O
,	O
which	O
tracks	O
very	O
recent	O
changes	O
that	O
might	O
not	O
have	O
been	O
fully	O
integrated	O
into	O
the	O
main	O
specification	O
.	O
</s>
<s>
The	O
Exim	B-Protocol
Specification	O
is	O
available	O
in	O
multiple	O
formats	O
,	O
including	O
online	O
in	O
HTML	O
and	O
in	O
plain-text	O
for	O
fast	O
searching	O
.	O
</s>
<s>
Exim	B-Protocol
has	O
been	O
deployed	O
in	O
busy	O
environments	O
,	O
often	O
handling	O
thousands	O
of	O
emails	O
per	O
hour	O
efficiently	O
.	O
</s>
<s>
Exim	B-Protocol
is	O
designed	O
to	O
deliver	O
email	O
immediately	O
,	O
without	O
queueing	O
.	O
</s>
<s>
Unlike	O
qmail	B-Application
,	O
Postfix	B-Application
,	O
and	O
ZMailer	B-Application
,	O
Exim	B-Protocol
does	O
not	O
have	O
a	O
central	O
queue	O
manager	O
(	O
i.e.	O
</s>
<s>
an	O
equivalent	O
of	O
qmail-send	O
,	O
qmgr	O
,	O
or	O
scheduler	O
)	O
.	O
</s>
<s>
System-wide	O
remote	O
transport	O
concurrency	O
is	O
unlimited	O
by	O
default	O
(	O
leading	O
to	O
a	O
"	O
thundering	B-Operating_System
herd	I-Operating_System
problem	I-Operating_System
"	O
when	O
multiple	O
messages	O
addressed	O
to	O
a	O
single	O
domain	O
are	O
submitted	O
at	O
once	O
)	O
but	O
can	O
be	O
limited	O
by	O
the	O
configuration	O
.	O
</s>
<s>
"	O
The	O
bottom	O
line	O
is	O
that	O
Exim	B-Protocol
does	O
not	O
perform	O
particularly	O
well	O
in	O
environments	O
where	O
the	O
queue	O
regularly	O
gets	O
very	O
large	O
.	O
</s>
<s>
In	O
1997	O
,	O
Hazel	O
replaced	O
Exim	B-Protocol
's	O
POSIX	O
regular	B-Language
expression	I-Language
library	O
written	O
by	O
Henry	O
Spencer	O
with	O
a	O
new	O
library	O
he	O
developed	O
called	O
PCRE	B-Language
(	O
Perl	B-Language
Compatible	I-Language
Regular	I-Language
Expressions	I-Language
)	O
.	O
</s>
<s>
Perl	B-Language
regular	B-Language
expressions	I-Language
are	O
much	O
more	O
powerful	O
than	O
POSIX	O
and	O
other	O
common	O
regular	B-Language
expressions	I-Language
,	O
and	O
PCRE	B-Language
has	O
become	O
popular	O
in	O
applications	O
other	O
than	O
Exim	B-Protocol
.	O
</s>
<s>
Exim	B-Protocol
transitioned	O
to	O
PCRE2	B-Language
.	O
</s>
<s>
Historically	O
,	O
Exim	B-Protocol
used	O
a	O
peculiar	O
version	O
numbering	O
scheme	O
where	O
the	O
first	O
decimal	O
digit	O
is	O
updated	O
only	O
whenever	O
the	O
main	O
documentation	O
is	O
fully	O
up	O
to	O
date	O
;	O
until	O
that	O
time	O
,	O
changes	O
were	O
accumulated	O
in	O
the	O
file	O
NewStuff	O
.	O
</s>
<s>
In	O
2005	O
,	O
changes	O
to	O
Exim	B-Protocol
's	O
version	O
numbering	O
were	O
on	O
the	O
table	O
of	O
discussion	O
.	O
</s>
<s>
In	O
more	O
recent	O
times	O
,	O
the	O
document	O
preparation	O
system	O
for	O
Exim	B-Protocol
has	O
been	O
overhauled	O
and	O
changes	O
are	O
much	O
more	O
likely	O
to	O
just	O
go	O
immediately	O
into	O
The	O
Exim	B-Protocol
Specification	O
.	O
</s>
<s>
Philip	O
Hazel	O
retired	O
from	O
the	O
University	O
of	O
Cambridge	O
in	O
2007	O
and	O
maintenance	O
of	O
Exim	B-Protocol
transitioned	O
to	O
a	O
team	O
of	O
maintainers	O
.	O
</s>
<s>
Exim	B-Protocol
continues	O
to	O
be	O
maintained	O
actively	O
,	O
with	O
frequent	O
releases	O
.	O
</s>
