<s>
An	O
email	B-Protocol
client	I-Protocol
,	O
email	O
reader	O
or	O
,	O
more	O
formally	O
,	O
message	B-Protocol
user	I-Protocol
agent	I-Protocol
(	O
MUA	O
)	O
or	O
mail	B-Protocol
user	I-Protocol
agent	I-Protocol
is	O
a	O
computer	B-Application
program	I-Application
used	O
to	O
access	O
and	O
manage	O
a	O
user	O
's	O
email	O
.	O
</s>
<s>
A	O
web	B-Application
application	I-Application
which	O
provides	O
message	O
management	O
,	O
composition	O
,	O
and	O
reception	O
functions	O
may	O
act	O
as	O
a	O
web	B-Protocol
email	I-Protocol
client	I-Protocol
,	O
and	O
a	O
piece	O
of	O
computer	B-Architecture
hardware	I-Architecture
or	O
software	O
whose	O
primary	O
or	O
most	O
visible	O
role	O
is	O
to	O
work	O
as	O
an	O
email	B-Protocol
client	I-Protocol
may	O
also	O
use	O
the	O
term	O
.	O
</s>
<s>
Like	O
most	O
client	O
programs	O
,	O
an	O
email	B-Protocol
client	I-Protocol
is	O
only	O
active	O
when	O
a	O
user	O
runs	O
it	O
.	O
</s>
<s>
(	O
MTA	B-Protocol
)	O
server	O
for	O
the	O
receipt	O
and	O
storage	O
of	O
the	O
client	O
's	O
emails	O
.	O
</s>
<s>
The	O
MTA	B-Protocol
,	O
using	O
a	O
suitable	O
mail	B-Protocol
delivery	I-Protocol
agent	I-Protocol
(	O
MDA	O
)	O
,	O
adds	O
email	O
messages	O
to	O
a	O
client	O
's	O
storage	O
as	O
they	O
arrive	O
.	O
</s>
<s>
The	O
default	O
setting	O
on	O
many	O
Unix	O
systems	O
is	O
for	O
the	O
mail	B-Protocol
server	I-Protocol
to	O
store	O
formatted	O
messages	O
in	O
mbox	B-Application
,	O
within	O
the	O
user	O
's	O
home	B-Application
directory	I-Application
.	O
</s>
<s>
Of	O
course	O
,	O
users	O
of	O
the	O
system	O
can	O
log-in	O
and	O
run	O
a	O
mail	B-Protocol
client	I-Protocol
on	O
the	O
same	O
computer	O
that	O
hosts	O
their	O
mailboxes	O
;	O
in	O
which	O
case	O
,	O
the	O
server	O
is	O
not	O
actually	O
remote	O
,	O
other	O
than	O
in	O
a	O
generic	O
sense	O
.	O
</s>
<s>
Emails	O
are	O
stored	O
in	O
the	O
user	O
's	O
mailbox	O
on	O
the	O
remote	O
server	O
until	O
the	O
user	O
's	O
email	B-Protocol
client	I-Protocol
requests	O
them	O
to	O
be	O
downloaded	O
to	O
the	O
user	O
's	O
computer	O
,	O
or	O
can	O
otherwise	O
access	O
the	O
user	O
's	O
mailbox	O
on	O
the	O
possibly	O
remote	O
server	O
.	O
</s>
<s>
The	O
email	B-Protocol
client	I-Protocol
can	O
be	O
set	O
up	O
to	O
connect	O
to	O
multiple	O
mailboxes	O
at	O
the	O
same	O
time	O
and	O
to	O
request	O
the	O
download	O
of	O
emails	O
either	O
automatically	O
,	O
such	O
as	O
at	O
pre-set	O
intervals	O
,	O
or	O
the	O
request	O
can	O
be	O
manually	O
initiated	O
by	O
the	O
user	O
.	O
</s>
<s>
The	O
Post	B-Protocol
Office	I-Protocol
Protocol	I-Protocol
(	O
POP	O
)	O
allows	O
the	O
user	O
to	O
download	O
messages	O
one	O
at	O
a	O
time	O
and	O
only	O
deletes	O
them	O
from	O
the	O
server	O
after	O
they	O
have	O
been	O
successfully	O
saved	O
on	O
local	O
storage	O
.	O
</s>
<s>
Alternatively	O
,	O
the	O
Internet	B-Protocol
Message	I-Protocol
Access	I-Protocol
Protocol	I-Protocol
(	O
IMAP	B-Protocol
)	O
allows	O
users	O
to	O
keep	O
messages	O
on	O
the	O
server	O
,	O
flagging	O
them	O
as	O
appropriate	O
.	O
</s>
<s>
IMAP	B-Protocol
provides	O
folders	O
and	O
sub-folders	O
,	O
which	O
can	O
be	O
shared	O
among	O
different	O
users	O
with	O
possibly	O
different	O
access	O
rights	O
.	O
</s>
<s>
IMAP	B-Protocol
features	O
an	O
idle	O
extension	O
for	O
real-time	O
updates	O
,	O
providing	O
faster	O
notification	O
than	O
polling	O
,	O
where	O
long-lasting	O
connections	O
are	O
feasible	O
.	O
</s>
<s>
The	O
JSON	B-Protocol
Meta	I-Protocol
Application	I-Protocol
Protocol	I-Protocol
(	O
JMAP	B-Protocol
)	O
is	O
implemented	O
using	O
JSON	O
APIs	O
over	O
HTTP	B-Protocol
and	O
has	O
been	O
developed	O
as	O
an	O
alternative	O
to	O
IMAP/SMTP	O
.	O
</s>
<s>
Direct	O
access	O
can	O
be	O
more	O
efficient	O
but	O
is	O
less	O
portable	O
as	O
it	O
depends	O
on	O
the	O
mailbox	O
format	O
;	O
it	O
is	O
used	O
by	O
some	O
email	B-Protocol
clients	I-Protocol
,	O
including	O
some	O
webmail	B-Protocol
applications	O
.	O
</s>
<s>
Email	B-Protocol
clients	I-Protocol
usually	O
contain	O
user	O
interfaces	O
to	O
display	O
and	O
edit	O
text	O
.	O
</s>
<s>
The	O
email	B-Protocol
clients	I-Protocol
will	O
perform	O
formatting	O
according	O
to	O
RFC	O
5322	O
for	O
headers	B-General_Concept
and	O
body	O
,	O
and	O
MIME	B-Protocol
for	O
non-textual	O
content	O
and	O
attachments	O
.	O
</s>
<s>
Headers	B-General_Concept
include	O
the	O
destination	O
fields	O
,	O
To	O
,	O
Cc	O
(	O
short	O
for	O
Carbon	O
copy	O
)	O
,	O
and	O
Bcc	O
(	O
Blind	O
carbon	O
copy	O
)	O
,	O
and	O
the	O
originator	O
fields	O
From	O
which	O
is	O
the	O
message	O
's	O
author(s )	O
,	O
Sender	O
in	O
case	O
there	O
are	O
more	O
authors	O
,	O
and	O
Reply-To	O
in	O
case	O
responses	O
should	O
be	O
addressed	O
to	O
a	O
different	O
mailbox	O
.	O
</s>
<s>
To	O
better	O
assist	O
the	O
user	O
with	O
destination	O
fields	O
,	O
many	O
clients	O
maintain	O
one	O
or	O
more	O
address	O
books	O
and/or	O
are	O
able	O
to	O
connect	O
to	O
an	O
LDAP	B-Protocol
directory	O
server	O
.	O
</s>
<s>
Client	O
settings	O
require	O
the	O
user	O
's	O
real	O
name	O
and	O
email	O
address	O
for	O
each	O
user	O
's	O
identity	O
,	O
and	O
possibly	O
a	O
list	O
of	O
LDAP	B-Protocol
servers	O
.	O
</s>
<s>
When	O
a	O
user	O
wishes	O
to	O
create	O
and	O
send	O
an	O
email	O
,	O
the	O
email	B-Protocol
client	I-Protocol
will	O
handle	O
the	O
task	O
.	O
</s>
<s>
The	O
email	B-Protocol
client	I-Protocol
is	O
usually	O
set	O
up	O
automatically	O
to	O
connect	O
to	O
the	O
user	O
's	O
mail	B-Protocol
server	I-Protocol
,	O
which	O
is	O
typically	O
either	O
an	O
MSA	B-Protocol
or	O
an	O
MTA	B-Protocol
,	O
two	O
variations	O
of	O
the	O
SMTP	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
The	O
email	B-Protocol
client	I-Protocol
which	O
uses	O
the	O
SMTP	B-Protocol
protocol	I-Protocol
creates	O
an	O
authentication	O
extension	O
,	O
which	O
the	O
mail	B-Protocol
server	I-Protocol
uses	O
to	O
authenticate	O
the	O
sender	O
.	O
</s>
<s>
The	O
older	O
method	O
was	O
for	O
the	O
mail	B-Protocol
server	I-Protocol
to	O
recognize	O
the	O
client	O
's	O
IP	O
address	O
,	O
e.g.	O
</s>
<s>
Client	O
settings	O
require	O
the	O
name	O
or	O
IP	O
address	O
of	O
the	O
preferred	O
outgoing	O
mail	B-Protocol
server	I-Protocol
,	O
the	O
port	O
number	O
(	O
25	O
for	O
MTA	B-Protocol
,	O
587	O
for	O
MSA	B-Protocol
)	O
,	O
and	O
the	O
user	O
name	O
and	O
password	O
for	O
the	O
authentication	O
,	O
if	O
any	O
.	O
</s>
<s>
There	O
is	O
a	O
non-standard	O
port	O
465	O
for	O
SSL	B-Protocol
encrypted	O
SMTP	B-Protocol
sessions	O
,	O
that	O
many	O
clients	O
and	O
servers	O
support	O
for	O
backward	O
compatibility	O
.	O
</s>
<s>
Email	B-Protocol
encryption	I-Protocol
enables	O
privacy	O
to	O
be	O
safeguarded	O
by	O
encrypting	O
the	O
mail	O
sessions	O
,	O
the	O
body	O
of	O
the	O
message	O
,	O
or	O
both	O
.	O
</s>
<s>
All	O
relevant	O
email	O
protocols	O
have	O
an	O
option	O
to	O
encrypt	O
the	O
whole	O
session	O
,	O
to	O
prevent	O
a	O
user	O
's	O
name	O
and	O
password	O
from	O
being	O
sniffed	B-Protocol
.	O
</s>
<s>
When	O
sending	O
mail	O
,	O
users	O
can	O
only	O
control	O
encryption	O
at	O
the	O
first	O
hop	O
from	O
a	O
client	O
to	O
its	O
configured	O
outgoing	O
mail	B-Protocol
server	I-Protocol
.	O
</s>
<s>
Encrypting	O
an	O
email	O
retrieval	O
session	O
with	O
,	O
e.g.	O
,	O
SSL	B-Protocol
,	O
can	O
protect	O
both	O
parts	O
(	O
authentication	O
,	O
and	O
message	O
transfer	O
)	O
of	O
the	O
session	O
.	O
</s>
<s>
Alternatively	O
,	O
if	O
the	O
user	O
has	O
SSH	B-Protocol
access	O
to	O
their	O
mail	B-Protocol
server	I-Protocol
,	O
they	O
can	O
use	O
SSH	B-Protocol
port	B-Protocol
forwarding	I-Protocol
to	O
create	O
an	O
encrypted	O
tunnel	O
over	O
which	O
to	O
retrieve	O
their	O
emails	O
.	O
</s>
<s>
S/MIME	B-Protocol
employs	O
a	O
model	O
based	O
on	O
a	O
trusted	O
certificate	B-Architecture
authority	I-Architecture
(	O
CA	O
)	O
that	O
signs	O
users	O
 '	O
public	O
keys	O
.	O
</s>
<s>
OpenPGP	O
is	O
also	O
more	O
flexible	O
in	O
the	O
format	O
of	O
the	O
messages	O
,	O
in	O
that	O
it	O
still	O
supports	O
plain	O
message	O
encryption	O
and	O
signing	O
as	O
they	O
used	O
to	O
work	O
before	O
MIME	B-Protocol
standardization	O
.	O
</s>
<s>
Header	B-General_Concept
fields	I-General_Concept
,	O
including	O
originator	O
,	O
recipients	O
,	O
and	O
often	O
subject	O
,	O
remain	O
in	O
plain	O
text	O
.	O
</s>
<s>
In	O
addition	O
to	O
email	B-Protocol
clients	I-Protocol
running	O
on	O
a	O
desktop	O
computer	O
,	O
there	O
are	O
those	O
hosted	O
remotely	O
,	O
either	O
as	O
part	O
of	O
a	O
remote	O
UNIX	O
installation	O
accessible	O
by	O
telnet	B-Protocol
(	O
i.e.	O
</s>
<s>
Both	O
of	O
these	O
approaches	O
have	O
several	O
advantages	O
:	O
they	O
share	O
an	O
ability	O
to	O
send	O
and	O
receive	O
email	O
away	O
from	O
the	O
user	O
's	O
normal	O
base	O
using	O
a	O
web	B-Application
browser	I-Application
or	O
telnet	B-Protocol
client	O
,	O
thus	O
eliminating	O
the	O
need	O
to	O
install	O
a	O
dedicated	O
email	B-Protocol
client	I-Protocol
on	O
the	O
user	O
's	O
device	O
.	O
</s>
<s>
Some	O
websites	O
are	O
dedicated	O
to	O
providing	O
email	O
services	O
,	O
and	O
many	O
Internet	O
service	O
providers	O
provide	O
webmail	B-Protocol
services	O
as	O
part	O
of	O
their	O
Internet	O
service	O
package	O
.	O
</s>
<s>
The	O
main	O
limitations	O
of	O
webmail	B-Protocol
are	O
that	O
user	O
interactions	O
are	O
subject	O
to	O
the	O
website	O
's	O
operating	O
system	O
and	O
the	O
general	O
inability	O
to	O
download	O
email	O
messages	O
and	O
compose	O
or	O
work	O
on	O
the	O
messages	O
offline	O
,	O
although	O
there	O
are	O
software	O
packages	O
that	O
can	O
integrate	O
parts	O
of	O
the	O
webmail	B-Protocol
functionality	O
into	O
the	O
OS	O
(	O
e.g.	O
</s>
<s>
creating	O
messages	O
directly	O
from	O
third	O
party	O
applications	O
via	O
MAPI	B-Protocol
)	O
.	O
</s>
<s>
Like	O
IMAP	B-Protocol
and	O
MAPI	B-Protocol
,	O
webmail	B-Protocol
provides	O
for	O
email	O
messages	O
to	O
remain	O
on	O
the	O
mail	B-Protocol
server	I-Protocol
.	O
</s>
<s>
POP3	B-Protocol
has	O
an	O
option	O
to	O
leave	O
messages	O
on	O
the	O
server	O
.	O
</s>
<s>
By	O
contrast	O
,	O
both	O
IMAP	B-Protocol
and	O
webmail	B-Protocol
keep	O
messages	O
on	O
the	O
server	O
as	O
their	O
method	O
of	O
operating	O
,	O
albeit	O
users	O
can	O
make	O
local	O
copies	O
as	O
they	O
like	O
.	O
</s>
<s>
With	O
limited	O
bandwidth	O
,	O
access	O
to	O
long	O
messages	O
can	O
be	O
lengthy	O
,	O
unless	O
the	O
email	B-Protocol
client	I-Protocol
caches	O
a	O
local	O
copy	O
.	O
</s>
<s>
Popular	O
protocols	O
for	O
retrieving	O
mail	O
include	O
POP3	B-Protocol
and	O
IMAP4	B-Protocol
.	O
</s>
<s>
Sending	O
mail	O
is	O
usually	O
done	O
using	O
the	O
SMTP	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
Another	O
important	O
standard	O
supported	O
by	O
most	O
email	B-Protocol
clients	I-Protocol
is	O
MIME	B-Protocol
,	O
which	O
is	O
used	O
to	O
send	O
binary	O
file	O
email	O
attachments	O
.	O
</s>
<s>
Most	O
email	B-Protocol
clients	I-Protocol
use	O
a	O
User-Agent	O
header	B-General_Concept
field	I-General_Concept
to	O
identify	O
the	O
software	O
used	O
to	O
send	O
the	O
message	O
.	O
</s>
<s>
According	O
to	O
RFC	O
2076	O
,	O
this	O
is	O
a	O
common	O
but	O
non-standard	O
header	B-General_Concept
field	I-General_Concept
.	O
</s>
<s>
RFC	O
6409	O
,	O
Message	O
Submission	O
for	O
Mail	O
,	O
details	O
the	O
role	O
of	O
the	O
Mail	B-Protocol
submission	I-Protocol
agent	I-Protocol
.	O
</s>
<s>
RFC	O
5068	O
,	O
Email	O
Submission	O
Operations	O
:	O
Access	O
and	O
Accountability	O
Requirements	O
,	O
provides	O
a	O
survey	O
of	O
the	O
concepts	O
of	O
MTA	B-Protocol
,	O
MSA	B-Protocol
,	O
MDA	O
,	O
and	O
MUA	O
.	O
</s>
<s>
It	O
mentions	O
that	O
"	O
Access	O
Providers	O
MUST	O
NOT	O
block	O
users	O
from	O
accessing	O
the	O
external	O
Internet	O
using	O
the	O
SUBMISSION	O
port	B-Protocol
587	I-Protocol
"	O
and	O
that	O
"	O
MUAs	O
SHOULD	O
use	O
the	O
SUBMISSION	O
port	O
for	O
message	O
submission.	O
"	O
</s>
<s>
Email	B-Protocol
servers	I-Protocol
and	O
clients	O
by	O
convention	O
use	O
the	O
TCP	B-Protocol
port	I-Protocol
numbers	I-Protocol
in	O
the	O
following	O
table	O
.	O
</s>
<s>
For	O
MSA	B-Protocol
,	O
IMAP	B-Protocol
and	O
POP3	B-Protocol
,	O
the	O
table	O
reports	O
also	O
the	O
labels	O
that	O
a	O
client	O
can	O
use	O
to	O
query	O
the	O
SRV	B-Protocol
records	I-Protocol
and	O
discover	O
both	O
the	O
host	O
name	O
and	O
the	O
port	O
number	O
of	O
the	O
corresponding	O
service	O
.	O
</s>
<s>
Note	O
that	O
while	O
webmail	B-Protocol
obeys	O
the	O
earlier	O
HTTP	B-Protocol
disposition	O
of	O
having	O
separate	O
ports	O
for	O
encrypt	O
and	O
plain	O
text	O
sessions	O
,	O
mail	B-Protocol
protocols	I-Protocol
use	O
the	O
STARTTLS	B-Protocol
technique	O
,	O
thereby	O
allowing	O
encryption	O
to	O
start	O
on	O
an	O
already	O
established	O
TCP	O
connection	O
.	O
</s>
<s>
While	O
RFC	O
2595	O
used	O
to	O
discourage	O
the	O
use	O
of	O
the	O
previously	O
established	O
ports	O
995	O
and	O
993	O
,	O
RFC	O
8314	O
promotes	O
the	O
use	O
of	O
implicit	O
TLS	B-Protocol
when	O
available	O
.	O
</s>
<s>
Microsoft	O
mail	O
systems	O
use	O
the	O
proprietary	B-Application
Messaging	B-Protocol
Application	I-Protocol
Programming	I-Protocol
Interface	I-Protocol
(	O
MAPI	B-Protocol
)	O
in	O
client	O
applications	O
,	O
such	O
as	O
Microsoft	B-Application
Outlook	I-Application
,	O
to	O
access	O
Microsoft	B-Operating_System
Exchange	I-Operating_System
electronic	O
mail	B-Protocol
servers	I-Protocol
.	O
</s>
