<s>
Within	O
the	O
Internet	O
email	O
system	O
,	O
a	O
message	O
transfer	O
agent	O
(	O
MTA	O
)	O
,	O
or	O
mail	B-Protocol
transfer	I-Protocol
agent	I-Protocol
,	O
or	O
mail	O
relay	O
is	O
software	O
that	O
transfers	O
electronic	O
mail	O
messages	O
from	O
one	O
computer	O
to	O
another	O
using	O
SMTP	B-Protocol
.	O
</s>
<s>
Access	O
to	O
this	O
email	O
by	O
end	O
users	O
is	O
typically	O
either	O
via	O
webmail	B-Protocol
or	O
an	O
email	B-Protocol
client	I-Protocol
.	O
</s>
<s>
A	O
message	O
transfer	O
agent	O
receives	O
mail	O
from	O
either	O
another	O
MTA	O
,	O
a	O
mail	B-Protocol
submission	I-Protocol
agent	I-Protocol
(	O
MSA	O
)	O
,	O
or	O
a	O
mail	B-Protocol
user	I-Protocol
agent	I-Protocol
(	O
MUA	O
)	O
.	O
</s>
<s>
The	O
transmission	O
details	O
are	O
specified	O
by	O
the	O
Simple	B-Protocol
Mail	I-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
(	O
SMTP	B-Protocol
)	O
.	O
</s>
<s>
The	O
process	O
of	O
choosing	O
a	O
target	O
MTA	O
for	O
the	O
next	O
hop	O
is	O
also	O
described	O
in	O
SMTP	B-Protocol
,	O
but	O
can	O
usually	O
be	O
overridden	O
by	O
configuring	O
the	O
MTA	O
software	O
with	O
specific	O
routes	O
.	O
</s>
<s>
An	O
MTA	O
works	O
in	O
the	O
background	O
,	O
while	O
the	O
user	O
usually	O
interacts	O
directly	O
with	O
a	O
mail	B-Protocol
user	I-Protocol
agent	I-Protocol
.	O
</s>
<s>
One	O
may	O
distinguish	O
initial	O
submission	O
as	O
first	O
passing	O
through	O
an	O
MSA	O
—	O
port	B-Protocol
587	I-Protocol
is	O
used	O
for	O
communication	O
between	O
an	O
MUA	O
and	O
an	O
MSA	O
,	O
while	O
port	B-Protocol
25	I-Protocol
is	O
used	O
for	O
communication	O
between	O
MTAs	O
,	O
or	O
from	O
an	O
MSA	O
to	O
an	O
MTA	O
;	O
this	O
distinction	O
is	O
first	O
made	O
in	O
RFC	O
2476	O
.	O
</s>
<s>
For	O
recipients	O
hosted	O
locally	O
,	O
the	O
final	O
delivery	O
of	O
email	O
to	O
a	O
recipient	O
mailbox	O
is	O
the	O
task	O
of	O
a	O
message	B-Protocol
delivery	I-Protocol
agent	I-Protocol
(	O
MDA	O
)	O
.	O
</s>
<s>
For	O
this	O
purpose	O
the	O
MTA	O
transfers	O
the	O
message	O
to	O
the	O
message	O
handling	O
service	O
component	O
of	O
the	O
message	B-Protocol
delivery	I-Protocol
agent	I-Protocol
(	O
MDA	O
)	O
.	O
</s>
<s>
A	O
relay	O
or	O
filtering	O
server	O
will	O
typically	O
store	O
email	O
only	O
briefly	O
,	O
but	O
other	O
systems	O
keep	O
full	O
mailboxes	O
for	O
email	O
-	O
in	O
which	O
case	O
they	O
usually	O
support	O
some	O
means	O
for	O
end	O
users	O
to	O
access	O
their	O
email	O
via	O
a	O
Mail	B-Protocol
User	I-Protocol
Agent	I-Protocol
(	O
MUA	O
)	O
,	O
or	O
email	B-Protocol
client	I-Protocol
.	O
</s>
<s>
Submission	O
of	O
new	O
email	O
from	O
a	O
mail	B-Protocol
client	I-Protocol
is	O
via	O
SMTP	B-Protocol
,	O
typically	O
on	O
port	B-Protocol
587	I-Protocol
or	O
465	O
,	O
and	O
is	O
now	O
generally	O
restricted	O
to	O
servers	O
the	O
user	O
has	O
an	O
account	O
with-such	O
as	O
their	O
ISP	O
.	O
</s>
