<s>
An	O
e-mail	B-Protocol
agent	I-Protocol
is	O
a	O
program	O
that	O
is	O
part	O
of	O
the	O
e-mail	O
infrastructure	O
,	O
from	O
composition	O
by	O
sender	O
,	O
to	O
transfer	O
across	O
the	O
network	O
,	O
to	O
viewing	O
by	O
recipient	O
.	O
</s>
<s>
The	O
best-known	O
are	O
message	B-Protocol
user	I-Protocol
agents	I-Protocol
(	O
MUAs	O
,	O
aka	O
,	O
e-mail	B-Protocol
clients	I-Protocol
)	O
and	O
message	B-Protocol
transfer	I-Protocol
agents	I-Protocol
(	O
MTAs	O
,	O
programs	O
that	O
transfer	O
e-mail	O
between	O
clients	O
)	O
,	O
but	O
finer	O
divisions	O
exist	O
.	O
</s>
<s>
More	O
precisely	O
,	O
this	O
is	O
a	O
technical	O
way	O
of	O
referring	O
to	O
functions	O
performed	O
by	O
various	O
programs	O
,	O
considering	O
them	O
as	O
software	B-General_Concept
agents	I-General_Concept
:	O
a	O
given	O
program	O
may	O
perform	O
several	O
functions	O
,	O
but	O
while	O
it	O
is	O
performing	O
a	O
given	O
function	O
(	O
such	O
as	O
message	O
transfer	O
)	O
,	O
it	O
is	O
referred	O
to	O
as	O
a	O
message	B-Protocol
transfer	I-Protocol
agent	I-Protocol
.	O
</s>
<s>
While	O
the	O
individual	O
terms	O
are	O
widely	O
used	O
in	O
internet	O
standards	O
and	O
RFCs	O
,	O
there	O
is	O
no	O
widely	O
used	O
umbrella	O
term	O
for	O
these	O
programs	O
,	O
though	O
such	O
a	O
program	O
may	O
informally	O
be	O
referred	O
to	O
generically	O
as	O
an	O
MxA	B-Protocol
,	O
'	O
x	O
 '	O
being	O
a	O
wildcard	B-Algorithm
,	O
as	O
the	O
programs	O
are	O
referred	O
to	O
by	O
acronyms	O
of	O
this	O
form	O
,	O
such	O
as	O
MUA	O
and	O
MTA	O
,	O
with	O
'	O
x	O
 '	O
depending	O
on	O
role	O
.	O
</s>
<s>
E-mail	B-Protocol
servers	I-Protocol
are	O
built	O
from	O
one	O
or	O
more	O
software	O
packages	O
,	O
each	O
of	O
which	O
carries	O
out	O
the	O
functions	O
of	O
one	O
or	O
more	O
MxA	B-Protocol
.	O
</s>
<s>
The	O
finest	O
and	O
most	O
expansive	O
classification	O
in	O
current	O
use	O
is	O
into	O
five	O
functions	O
in	O
addition	O
to	O
the	O
mail	B-Protocol
exchanger	I-Protocol
(	O
MX	O
)	O
:	O
</s>
<s>
Mail	B-Protocol
retrieval	I-Protocol
agent	I-Protocol
(	O
MRA	O
)	O
(	O
Unlike	O
the	O
above	O
terms	O
,	O
this	O
term	O
is	O
not	O
supported	O
by	O
IETF	O
documents	O
,	O
in	O
which	O
it	O
is	O
considered	O
an	O
MUA	O
.	O
)	O
</s>
<s>
The	O
traditional	O
division	O
is	O
into	O
client-side	O
(	O
MUA	O
)	O
and	O
server-side	O
(	O
MTA	O
,	O
notably	O
sendmail	B-Application
)	O
,	O
with	O
the	O
flow	O
given	O
as	O
:	O
</s>
<s>
Programs	O
such	O
as	O
fetchmail	B-Protocol
which	O
retrieve	O
mail	O
from	O
a	O
server	O
but	O
do	O
not	O
provide	O
a	O
human	O
interface	O
for	O
viewing	O
or	O
other	O
client	O
tasks	O
are	O
referred	O
to	O
as	O
MRAs	O
–	O
they	O
provide	O
retrieval	O
but	O
no	O
other	O
client	O
functions	O
.	O
</s>
<s>
However	O
,	O
traditional	O
Unix	O
mail	B-Protocol
readers	I-Protocol
such	O
as	O
elm	O
,	O
Pine	O
,	O
or	O
mutt	O
would	O
often	O
not	O
include	O
MRA	O
functions	O
(	O
or	O
only	O
optionally	O
)	O
,	O
reading	O
mail	O
that	O
had	O
already	O
been	O
delivered	O
to	O
a	O
mailbox	O
file	O
(	O
formally	O
,	O
delivered	O
by	O
an	O
MDA	O
)	O
.	O
</s>
<s>
The	O
delivery	O
(	O
MDA	O
)	O
stage	O
is	O
where	O
such	O
tasks	O
as	O
filtering	O
(	O
of	O
undesired	O
messages	O
)	O
and	O
filing	O
(	O
into	O
separate	O
folders	O
)	O
occur	O
,	O
and	O
are	O
the	O
start	O
of	O
the	O
user	O
agent	O
;	O
traditionally	O
this	O
was	O
done	O
via	O
procmail	B-Protocol
,	O
while	O
today	O
it	O
may	O
be	O
done	O
via	O
server-side	O
programs	O
,	O
often	O
using	O
spam	O
filters	O
such	O
as	O
SpamAssassin	B-Language
.	O
</s>
