<s>
In	O
computing	O
,	O
the	O
Post	B-Protocol
Office	I-Protocol
Protocol	I-Protocol
(	O
POP	O
)	O
is	O
an	O
application-layer	B-Application
Internet	O
standard	O
protocol	O
used	O
by	O
e-mail	B-Protocol
clients	I-Protocol
to	O
retrieve	O
e-mail	O
from	O
a	O
mail	B-Protocol
server	I-Protocol
.	O
</s>
<s>
POP	B-Protocol
version	I-Protocol
3	I-Protocol
(	O
POP3	B-Protocol
)	O
is	O
the	O
version	O
in	O
common	O
use	O
,	O
and	O
along	O
with	O
IMAP	B-Protocol
the	O
most	O
common	O
protocols	O
for	O
email	O
retrieval	O
.	O
</s>
<s>
The	O
Post	B-Protocol
Office	I-Protocol
Protocol	I-Protocol
provides	O
access	O
via	O
an	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
network	O
for	O
a	O
user	O
client	O
application	O
to	O
a	O
mailbox	O
(	O
maildrop	O
)	O
maintained	O
on	O
a	O
mail	B-Protocol
server	I-Protocol
.	O
</s>
<s>
POP3	B-Protocol
clients	O
connect	O
,	O
retrieve	O
all	O
messages	O
,	O
store	O
them	O
on	O
the	O
client	O
computer	O
,	O
and	O
finally	O
delete	O
them	O
from	O
the	O
server	B-Protocol
.	O
</s>
<s>
POP3	B-Protocol
clients	O
also	O
have	O
an	O
option	O
to	O
leave	O
mail	O
on	O
the	O
server	B-Protocol
after	O
download	O
.	O
</s>
<s>
By	O
contrast	O
,	O
the	O
Internet	B-Protocol
Message	I-Protocol
Access	I-Protocol
Protocol	I-Protocol
(	O
IMAP	B-Protocol
)	O
was	O
designed	O
to	O
normally	O
leave	O
all	O
messages	O
on	O
the	O
server	B-Protocol
to	O
permit	O
management	O
with	O
multiple	O
client	O
applications	O
,	O
and	O
to	O
support	O
both	O
connected	O
(	O
online	O
)	O
and	O
disconnected	O
(	O
offline	O
)	O
modes	O
of	O
operation	O
.	O
</s>
<s>
A	O
POP3	B-Protocol
server	B-Protocol
listens	O
on	O
well-known	B-Protocol
port	I-Protocol
number	I-Protocol
110	O
for	O
service	O
requests	O
.	O
</s>
<s>
Encrypted	O
communication	O
for	O
POP3	B-Protocol
is	O
either	O
requested	O
after	O
protocol	O
initiation	O
,	O
using	O
the	O
STLS	B-Protocol
command	O
,	O
if	O
supported	O
,	O
or	O
by	O
POP3S	B-Protocol
,	O
which	O
connects	O
to	O
the	O
server	B-Protocol
using	O
Transport	B-Protocol
Layer	I-Protocol
Security	I-Protocol
(	O
TLS	O
)	O
or	O
Secure	B-Protocol
Sockets	I-Protocol
Layer	I-Protocol
(	O
SSL	O
)	O
on	O
well-known	O
TCP	B-Protocol
port	I-Protocol
number	O
995	O
.	O
</s>
<s>
Messages	O
available	O
to	O
the	O
client	O
are	O
determined	O
when	O
a	O
POP3	B-Protocol
session	B-Protocol
opens	O
the	O
maildrop	O
,	O
and	O
are	O
identified	O
by	O
message-number	O
local	O
to	O
that	O
session	B-Protocol
or	O
,	O
optionally	O
,	O
by	O
a	O
unique	O
identifier	O
assigned	O
to	O
the	O
message	O
by	O
the	O
POP	B-Protocol
server	I-Protocol
.	O
</s>
<s>
When	O
the	O
client	O
exits	O
the	O
session	B-Protocol
,	O
mail	O
marked	O
for	O
deletion	O
is	O
removed	O
from	O
the	O
maildrop	O
.	O
</s>
<s>
The	O
first	O
version	O
of	O
the	O
Post	B-Protocol
Office	I-Protocol
Protocol	I-Protocol
,	O
POP1	O
,	O
was	O
specified	O
in	O
RFC	O
918	O
(	O
1984	O
)	O
by	O
Joyce	O
K	O
.	O
Reynolds	O
.	O
</s>
<s>
POP3	B-Protocol
is	O
the	O
version	O
in	O
most	O
common	O
use	O
.	O
</s>
<s>
This	O
led	O
to	O
a	O
number	O
of	O
POP	O
implementations	O
such	O
as	O
Pine	O
,	O
POPmail	B-Operating_System
,	O
and	O
other	O
early	O
mail	B-Protocol
clients	I-Protocol
.	O
</s>
<s>
While	O
the	O
original	O
POP3	B-Protocol
specification	O
supported	O
only	O
an	O
unencrypted	O
USER/PASS	O
login	B-General_Concept
mechanism	O
or	O
Berkeley	O
.rhosts	B-Application
access	O
control	O
,	O
today	O
POP3	B-Protocol
supports	O
several	O
authentication	O
methods	O
to	O
provide	O
varying	O
levels	O
of	O
protection	O
against	O
illegitimate	O
access	O
to	O
a	O
user	O
's	O
e-mail	O
.	O
</s>
<s>
Most	O
are	O
provided	O
by	O
the	O
POP3	B-Protocol
extension	O
mechanisms	O
.	O
</s>
<s>
POP3	B-Protocol
clients	O
support	O
SASL	O
authentication	O
methods	O
via	O
the	O
AUTH	O
extension	O
.	O
</s>
<s>
MIT	O
Project	B-Operating_System
Athena	I-Operating_System
also	O
produced	O
a	O
Kerberized	B-Protocol
version	O
.	O
</s>
<s>
RFC	O
1460	O
introduced	O
APOP	B-Protocol
into	O
the	O
core	O
protocol	O
.	O
</s>
<s>
APOP	B-Protocol
is	O
a	O
challenge	O
–	O
response	O
protocol	O
which	O
uses	O
the	O
MD5	B-Algorithm
hash	I-Algorithm
function	O
in	O
an	O
attempt	O
to	O
avoid	O
replay	O
attacks	O
and	O
disclosure	O
of	O
the	O
shared	O
secret	O
.	O
</s>
<s>
Clients	O
implementing	O
APOP	B-Protocol
include	O
Mozilla	B-Application
Thunderbird	I-Application
,	O
Opera	B-Operating_System
Mail	I-Operating_System
,	O
Eudora	B-Operating_System
,	O
KMail	O
,	O
Novell	B-Language
Evolution	I-Language
,	O
RimArts	O
 '	O
Becky	O
!,	O
Windows	B-Application
Live	I-Application
Mail	I-Application
,	O
PowerMail	O
,	O
Apple	B-Operating_System
Mail	I-Operating_System
,	O
and	O
Mutt	B-Device
.	O
</s>
<s>
POP4	O
exists	O
only	O
as	O
an	O
informal	O
proposal	O
adding	O
basic	O
folder	O
management	O
,	O
multipart	B-Protocol
message	I-Protocol
support	O
,	O
as	O
well	O
as	O
message	O
flag	O
management	O
to	O
compete	O
with	O
IMAP	B-Protocol
;	O
however	O
,	O
its	O
development	O
has	O
not	O
progressed	O
since	O
2003	O
.	O
</s>
<s>
An	O
extension	O
mechanism	O
was	O
proposed	O
in	O
RFC	O
2449	O
to	O
accommodate	O
general	O
extensions	O
as	O
well	O
as	O
announce	O
in	O
an	O
organized	O
manner	O
support	O
for	O
optional	O
commands	O
,	O
such	O
as	O
TOP	O
and	O
UIDL	B-Protocol
.	O
</s>
<s>
The	O
RFC	O
did	O
not	O
intend	O
to	O
encourage	O
extensions	O
,	O
and	O
reaffirmed	O
that	O
the	O
role	O
of	O
POP3	B-Protocol
is	O
to	O
provide	O
simple	O
support	O
for	O
mainly	O
download-and-delete	O
requirements	O
of	O
mailbox	O
handling	O
.	O
</s>
<s>
With	O
the	O
exception	O
of	O
APOP	B-Protocol
,	O
the	O
optional	O
commands	O
were	O
included	O
in	O
the	O
initial	O
set	O
of	O
capabilities	O
.	O
</s>
<s>
The	O
STARTTLS	B-Protocol
extension	O
allows	O
the	O
use	O
of	O
Transport	B-Protocol
Layer	I-Protocol
Security	I-Protocol
(	O
TLS	O
)	O
or	O
Secure	B-Protocol
Sockets	I-Protocol
Layer	I-Protocol
(	O
SSL	O
)	O
to	O
be	O
negotiated	O
using	O
the	O
STLS	B-Protocol
command	O
,	O
on	O
the	O
standard	O
POP3	B-Protocol
port	B-Protocol
,	O
rather	O
than	O
an	O
alternate	O
.	O
</s>
<s>
Some	O
clients	O
and	O
servers	O
instead	O
use	O
the	O
alternate-port	O
method	O
,	O
which	O
uses	O
TCP	B-Protocol
port	I-Protocol
995	O
(	O
POP3S	B-Protocol
)	O
.	O
</s>
<s>
Demon	O
Internet	O
introduced	O
extensions	O
to	O
POP3	B-Protocol
that	O
allow	O
multiple	O
accounts	O
per	O
domain	O
,	O
and	O
has	O
become	O
known	O
as	O
Standard	B-Protocol
Dial-up	I-Protocol
POP3	I-Protocol
Service	I-Protocol
(	O
SDPS	O
)	O
.	O
</s>
<s>
In	O
computing	O
,	O
local	O
e-mail	B-Protocol
clients	I-Protocol
can	O
use	O
the	O
Kerberized	B-Protocol
Post	B-Protocol
Office	I-Protocol
Protocol	I-Protocol
(	O
KPOP	O
)	O
,	O
an	O
application-layer	B-Application
Internet	O
standard	O
protocol	O
,	O
to	O
retrieve	O
e-mail	O
from	O
a	O
remote	O
server	B-Protocol
over	O
a	O
TCP/IP	B-Protocol
connection	O
.	O
</s>
<s>
The	O
KPOP	O
protocol	O
is	O
based	O
on	O
the	O
POP3	B-Protocol
protocol	O
–	O
differing	O
in	O
that	O
it	O
adds	O
Kerberos	B-Protocol
security	O
and	O
that	O
it	O
runs	O
by	O
default	O
over	O
TCP	B-Protocol
port	I-Protocol
number	O
1109	O
instead	O
of	O
110	O
.	O
</s>
<s>
One	O
mail	B-Protocol
server	I-Protocol
software	O
implementation	O
is	O
found	O
in	O
the	O
Cyrus	B-Protocol
IMAP	I-Protocol
server	I-Protocol
.	O
</s>
<s>
The	O
following	O
POP3	B-Protocol
session	B-Protocol
dialog	O
is	O
an	O
example	O
in	O
RFC	O
1939	O
:	O
</s>
<s>
POP3	B-Protocol
servers	O
without	O
the	O
optional	O
APOP	B-Protocol
command	O
expect	O
the	O
client	O
to	O
log	O
in	O
with	O
the	O
USER	O
and	O
PASS	O
commands	O
:	O
</s>
<s>
The	O
Internet	B-Protocol
Message	I-Protocol
Access	I-Protocol
Protocol	I-Protocol
(	O
IMAP	B-Protocol
)	O
is	O
an	O
alternative	O
and	O
more	O
recent	O
mailbox	O
access	O
protocol	O
.	O
</s>
<s>
POP	O
moves	O
the	O
message	O
from	O
the	O
email	B-Protocol
server	I-Protocol
to	O
the	O
local	O
computer	O
,	O
although	O
there	O
is	O
usually	O
an	O
option	O
in	O
email	B-Protocol
clients	I-Protocol
to	O
leave	O
the	O
messages	O
on	O
the	O
email	B-Protocol
server	I-Protocol
as	O
well	O
.	O
</s>
<s>
IMAP	B-Protocol
defaults	O
to	O
leaving	O
the	O
message	O
on	O
the	O
email	B-Protocol
server	I-Protocol
,	O
simply	O
downloading	O
a	O
local	O
copy	O
.	O
</s>
<s>
An	O
IMAP	B-Protocol
client	O
performs	O
complex	O
queries	O
,	O
asking	O
the	O
server	B-Protocol
for	O
headers	O
,	O
or	O
the	O
bodies	O
of	O
specified	O
messages	O
,	O
or	O
to	O
search	O
for	O
messages	O
meeting	O
certain	O
criteria	O
.	O
</s>
<s>
"	O
deleted	O
"	O
or	O
"	O
answered	O
"	O
)	O
and	O
they	O
stay	O
in	O
the	O
repository	O
until	O
explicitly	O
removed	O
by	O
the	O
user	O
—	O
which	O
may	O
not	O
be	O
until	O
a	O
later	O
session	B-Protocol
.	O
</s>
<s>
In	O
short	O
:	O
IMAP	B-Protocol
is	O
designed	O
to	O
permit	O
manipulation	O
of	O
remote	O
mailboxes	O
as	O
if	O
they	O
were	O
local	O
.	O
</s>
<s>
Depending	O
on	O
the	O
IMAP	B-Protocol
client	O
implementation	O
and	O
the	O
mail	O
architecture	O
desired	O
by	O
the	O
system	O
manager	O
,	O
the	O
user	O
may	O
save	O
messages	O
directly	O
on	O
the	O
client	O
machine	O
,	O
or	O
save	O
them	O
on	O
the	O
server	B-Protocol
,	O
or	O
be	O
given	O
the	O
choice	O
of	O
doing	O
either	O
.	O
</s>
<s>
POP	O
provides	O
a	O
completely	O
static	O
view	O
of	O
the	O
current	O
state	O
of	O
the	O
mailbox	O
,	O
and	O
does	O
not	O
provide	O
a	O
mechanism	O
to	O
show	O
any	O
external	O
changes	O
in	O
state	O
during	O
the	O
session	B-Protocol
.	O
</s>
<s>
IMAP	B-Protocol
provides	O
a	O
dynamic	O
view	O
,	O
and	O
sends	O
responses	O
for	O
external	O
changes	O
in	O
state	O
,	O
including	O
newly	O
arrived	O
messages	O
,	O
as	O
well	O
as	O
changes	O
made	O
to	O
the	O
mailbox	O
by	O
other	O
concurrently	O
connected	O
clients	O
.	O
</s>
<s>
IMAP	B-Protocol
allows	O
clients	O
to	O
retrieve	O
any	O
of	O
the	O
individual	O
MIME	B-Protocol
parts	O
separately	O
–	O
for	O
example	O
,	O
retrieving	O
the	O
plain	O
text	O
without	O
retrieving	O
attached	O
files	O
,	O
or	O
retrieving	O
only	O
one	O
of	O
many	O
attached	O
files	O
.	O
</s>
<s>
IMAP	B-Protocol
supports	O
flags	O
on	O
the	O
server	B-Protocol
to	O
keep	O
track	O
of	O
message	O
state	O
:	O
for	O
example	O
,	O
whether	O
or	O
not	O
the	O
message	O
has	O
been	O
read	O
,	O
replied	O
to	O
,	O
forwarded	O
,	O
or	O
deleted	O
.	O
</s>
