<s>
A	O
message	B-Protocol
submission	I-Protocol
agent	I-Protocol
(	O
MSA	O
)	O
,	O
or	O
mail	B-Protocol
submission	I-Protocol
agent	I-Protocol
,	O
is	O
a	O
computer	B-Application
program	I-Application
or	O
software	B-General_Concept
agent	I-General_Concept
that	O
receives	O
electronic	O
mail	O
messages	O
from	O
a	O
mail	B-Protocol
user	I-Protocol
agent	I-Protocol
(	O
MUA	O
)	O
and	O
cooperates	O
with	O
a	O
mail	B-Protocol
transfer	I-Protocol
agent	I-Protocol
(	O
MTA	B-Protocol
)	O
for	O
delivery	O
of	O
the	O
mail	O
.	O
</s>
<s>
It	O
uses	O
ESMTP	B-Protocol
,	O
a	O
variant	O
of	O
the	O
Simple	B-Protocol
Mail	I-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
(	O
SMTP	B-Protocol
)	O
,	O
as	O
specified	O
in	O
RFC	O
6409	O
.	O
</s>
<s>
Many	O
MTAs	O
perform	O
the	O
function	O
of	O
an	O
MSA	O
as	O
well	O
,	O
but	O
there	O
are	O
also	O
programs	O
that	O
are	O
specially	O
designed	O
as	O
MSAs	O
without	O
full	O
MTA	B-Protocol
functionality	O
.	O
</s>
<s>
Historically	O
,	O
in	O
Internet	O
mail	O
,	O
both	O
MTA	B-Protocol
and	O
MSA	O
functions	O
use	O
port	B-Protocol
number	I-Protocol
25	O
,	O
but	O
the	O
official	O
port	O
for	O
MSAs	O
is	O
587	O
.	O
</s>
<s>
The	O
MTA	B-Protocol
accepts	O
a	O
user	O
's	O
incoming	O
mail	O
,	O
while	O
the	O
MSA	O
accepts	O
a	O
user	O
's	O
outgoing	O
mail	O
.	O
</s>
<s>
Separation	O
of	O
the	O
MTA	B-Protocol
and	O
MSA	O
functions	O
produces	O
several	O
benefits	O
.	O
</s>
<s>
An	O
MTA	B-Protocol
accepting	O
a	O
message	O
from	O
another	O
site	O
cannot	O
reliably	O
make	O
those	O
kinds	O
of	O
corrections	O
,	O
and	O
any	O
error	O
reports	O
generated	O
by	O
such	O
an	O
MTA	B-Protocol
will	O
reach	O
the	O
author	O
(	O
if	O
at	O
all	O
)	O
only	O
after	O
the	O
message	O
has	O
already	O
been	O
sent	O
.	O
</s>
<s>
One	O
more	O
benefit	O
is	O
that	O
with	O
a	O
dedicated	O
port	B-Protocol
number	I-Protocol
,	O
587	O
,	O
it	O
is	O
always	O
possible	O
for	O
users	O
to	O
connect	O
to	O
their	O
domain	O
to	O
submit	O
new	O
mail	O
.	O
</s>
<s>
To	O
combat	O
spam	O
(	O
including	O
spam	O
being	O
sent	O
unwittingly	O
by	O
a	O
victim	O
of	O
a	O
botnet	B-Architecture
)	O
many	O
ISPs	O
and	O
institutional	O
networks	B-Architecture
restrict	O
the	O
ability	O
to	O
connect	O
to	O
remote	O
MTAs	O
on	O
port	B-Protocol
25	I-Protocol
.	O
</s>
<s>
The	O
accessibility	O
of	O
an	O
MSA	O
on	O
port	B-Protocol
587	I-Protocol
enables	O
nomadic	O
users	O
(	O
for	O
example	O
,	O
those	O
working	O
on	O
a	O
laptop	O
)	O
to	O
continue	O
to	O
send	O
mail	O
via	O
their	O
preferred	O
submission	O
servers	O
even	O
from	O
within	O
others	O
 '	O
networks	B-Architecture
.	O
</s>
<s>
Using	O
a	O
specific	O
submission	O
server	O
is	O
a	O
requirement	O
when	O
sender	B-Protocol
policies	I-Protocol
or	O
signing	O
practices	O
are	O
enforced	O
.	O
</s>
<s>
Another	O
benefit	O
is	O
that	O
separating	O
the	O
MTA	B-Protocol
and	O
MSA	O
functions	O
makes	O
it	O
easier	O
for	O
an	O
MTA	B-Protocol
to	O
deny	O
relaying	O
,	O
that	O
is	O
to	O
refuse	O
any	O
mail	O
that	O
is	O
not	O
addressed	O
to	O
a	O
recipient	O
at	O
a	O
domain	O
that	O
is	O
served	O
locally	O
.	O
</s>
<s>
In	O
times	O
when	O
both	O
mail	O
submission	O
and	O
acceptance	O
of	O
incoming	O
mail	O
were	O
usually	O
accomplished	O
using	O
the	O
same	O
protocol	O
and	O
the	O
same	O
server	O
,	O
the	O
ability	O
to	O
send	O
mail	O
to	O
arbitrary	O
destinations	O
without	O
authentication	O
allowed	O
spammers	O
to	O
use	O
MTAs	O
as	O
a	O
means	O
of	O
distributing	O
spam	O
(	O
since	O
a	O
single	O
message	O
transaction	O
can	O
request	O
that	O
an	O
MTA	B-Protocol
relay	O
a	O
message	O
to	O
a	O
large	O
number	O
of	O
recipients	O
)	O
,	O
and	O
also	O
made	O
it	O
more	O
difficult	O
to	O
trace	O
a	O
message	O
to	O
its	O
origin	O
.	O
</s>
<s>
This	O
allows	O
the	O
MSA	O
to	O
have	O
either	O
no	O
spam	O
filtering	O
,	O
or	O
more	O
permissive	O
spam	O
filtering	O
than	O
an	O
MTA	B-Protocol
that	O
exists	O
for	O
the	O
purpose	O
of	O
accepting	O
incoming	O
email	O
from	O
other	O
domains	O
.	O
</s>
<s>
In	O
the	O
absence	O
of	O
such	O
trust	O
,	O
an	O
MTA	B-Protocol
must	O
generally	O
rely	O
on	O
heuristics	O
and	O
third-party	O
reputation	O
services	O
to	O
distinguish	O
spam	O
from	O
legitimate	O
traffic	O
,	O
and	O
both	O
of	O
these	O
mechanisms	O
have	O
a	O
history	O
of	O
being	O
error-prone	O
.	O
</s>
<s>
The	O
separation	O
of	O
MSA	O
and	O
MTA	B-Protocol
therefore	O
avoids	O
the	O
use	O
of	O
unreliable	O
spam	O
recognition	O
mechanisms	O
during	O
mail	O
submission	O
,	O
and	O
increases	O
the	O
probability	O
for	O
legitimate	O
mail	O
to	O
be	O
delivered	O
successfully	O
.	O
</s>
<s>
While	O
recent	O
email	B-Protocol
clients	I-Protocol
use	O
port	B-Protocol
587	I-Protocol
by	O
default	O
,	O
older	O
ones	O
still	O
propose	O
port	B-Protocol
25	I-Protocol
.	O
</s>
<s>
Users	O
have	O
to	O
change	O
the	O
port	B-Protocol
number	I-Protocol
manually	O
in	O
the	O
latter	O
case	O
.	O
</s>
<s>
It	O
is	O
also	O
possible	O
that	O
the	O
MUA	O
may	O
automatically	O
discover	O
which	O
server	O
provides	O
the	O
MSA	O
for	O
a	O
given	O
domain	O
,	O
looking	O
up	O
the	O
SRV	B-Protocol
records	I-Protocol
for	O
that	O
domain	O
.	O
</s>
<s>
RFC	O
6409	O
requires	O
that	O
clients	O
are	O
authorized	O
and	O
authenticated	O
to	O
use	O
the	O
mail	O
submission	O
service	O
,	O
e.g.	O
,	O
as	O
described	O
in	O
SMTP-AUTH	O
(	O
ESMTPA	O
)	O
,	O
or	O
by	O
other	O
means	O
such	O
as	O
RADIUS	B-Protocol
,	O
public	O
key	O
certificates	O
,	O
or	O
(	O
the	O
mostly	O
obsolete	O
)	O
POP	O
before	O
SMTP	B-Protocol
.	O
</s>
<s>
This	O
in	O
essence	O
complies	O
with	O
the	O
SPF	B-Protocol
model	O
specified	O
in	O
RFC	O
7208	O
.	O
</s>
