<s>
The	O
Courier	B-Protocol
Mail	I-Protocol
Server	I-Protocol
is	O
a	O
mail	B-Protocol
transfer	I-Protocol
agent	I-Protocol
(	O
MTA	O
)	O
server	O
that	O
provides	O
SMTP	B-Protocol
,	O
IMAP	B-Protocol
,	O
POP3	B-Protocol
,	O
SMAP	B-Protocol
,	O
webmail	B-Protocol
,	O
and	O
mailing	O
list	O
services	O
with	O
individual	O
components	O
.	O
</s>
<s>
It	O
is	O
best	O
known	O
for	O
its	O
IMAP	B-Protocol
server	I-Protocol
component	O
.	O
</s>
<s>
Courier	O
can	O
function	O
as	O
an	O
intermediate	O
mail	B-Protocol
relay	I-Protocol
,	O
between	O
an	O
internal	O
LAN	O
and	O
the	O
Internet	O
,	O
or	O
perform	O
final	O
delivery	O
to	O
mailboxes	O
.	O
</s>
<s>
Configuration	O
files	O
are	O
in	O
plain	O
text	O
format	O
and	O
may	O
include	O
Perl	B-Language
scripts	O
.	O
</s>
<s>
Courier	O
can	O
also	O
provide	O
mail	O
services	O
for	O
virtual	O
mail	O
accounts	O
,	O
managed	O
by	O
any	O
of	O
LDAP	B-Protocol
directory	O
service	O
,	O
Berkeley	B-Language
DB	I-Language
,	O
MySQL	B-Application
or	O
PostgreSQL	B-Application
authentication	O
database	O
.	O
</s>
<s>
Parts	O
of	O
Courier	O
,	O
such	O
as	O
the	O
maildrop	B-Protocol
filtering	O
system	O
,	O
the	O
webmail	B-Protocol
and	O
IMAP	B-Protocol
server	I-Protocol
,	O
can	O
also	O
be	O
installed	O
as	O
independent	O
packages	O
which	O
can	O
be	O
used	O
with	O
other	O
mail	B-Protocol
servers	I-Protocol
.	O
</s>
<s>
Courier-IMAP	O
is	O
a	O
particularly	O
popular	O
combination	O
with	O
Qmail	B-Application
,	O
Exim	B-Protocol
,	O
and	O
Postfix	B-Application
servers	O
that	O
are	O
configured	O
to	O
use	O
maildirs	O
.	O
</s>
<s>
Courier	O
's	O
source	O
compiles	O
on	O
most	O
POSIX-based	O
operating	O
systems	O
based	O
on	O
Linux	B-Application
and	O
BSD-derived	O
kernels	O
.	O
</s>
<s>
It	O
uses	O
SMTP	B-Protocol
extensions	O
for	O
list	O
management	O
and	O
spam	O
filtering	O
.	O
</s>
