<s>
On-Demand	B-Protocol
Mail	I-Protocol
Relay	I-Protocol
(	O
ODMR	O
)	O
is	O
an	O
SMTP	B-Protocol
extension	O
standardized	O
in	O
RFC	O
2645	O
that	O
allows	O
e-mail	O
to	O
be	O
relayed	O
to	O
the	O
recipients	O
after	O
they	O
have	O
been	O
authenticated	O
.	O
</s>
<s>
It	O
uses	O
the	O
extended	B-Protocol
SMTP	I-Protocol
command	O
ATRN	O
,	O
similar	O
to	O
the	O
ETRN	B-Protocol
command	O
but	O
available	O
for	O
dynamically	O
assigned	O
IP	O
addresses	O
.	O
</s>
<s>
ODMR	O
works	O
in	O
the	O
following	O
way	O
:	O
to	O
check	O
their	O
inbound	O
mail	O
,	O
customers	O
issue	O
EHLO	B-Protocol
and	O
AUTH	O
commands	O
to	O
the	O
ODMR	O
service	O
,	O
followed	O
by	O
ATRN	O
command	O
(	O
Authenticated	O
TuRN	O
)	O
;	O
after	O
a	O
successful	O
ATRN	O
command	O
the	O
ODMR	O
server	O
begins	O
to	O
act	O
as	O
an	O
SMTP	B-Protocol
client	O
and	O
starts	O
to	O
send	O
all	O
messages	O
directed	O
to	O
the	O
customer	O
's	O
domain	O
(	O
or	O
domains	O
)	O
using	O
SMTP	B-Protocol
protocol	I-Protocol
,	O
via	O
the	O
same	O
TCP	O
connection	O
.	O
</s>
<s>
your	O
ISP	O
may	O
block	O
incoming	O
connection	O
on	O
port	B-Protocol
25	I-Protocol
or	O
may	O
use	O
NAT	B-Protocol
)	O
.	O
</s>
<s>
Only	O
the	O
ODMR	O
server	O
,	O
referred	O
as	O
the	O
provider	O
,	O
must	O
listen	O
to	O
SMTP	B-Protocol
connections	O
on	O
a	O
fixed	O
IP	O
address	O
.	O
</s>
<s>
The	O
design	O
of	O
ODMR	O
allows	O
the	O
protocol	O
to	O
be	O
used	O
with	O
an	O
SMTP	B-Protocol
server	I-Protocol
that	O
does	O
n't	O
support	O
it	O
natively	O
.	O
</s>
<s>
A	O
separate	O
application	O
can	O
be	O
used	O
to	O
do	O
the	O
initial	O
ODMR	O
protocol	O
negotiation	O
(	O
EHLO	B-Protocol
,	O
AUTH	O
and	O
ATRN	O
)	O
,	O
then	O
hand	O
over	O
the	O
connection	O
to	O
the	O
SMTP	B-Protocol
server	I-Protocol
once	O
the	O
protocol	O
has	O
reached	O
the	O
"	O
reversed	O
"	O
state	O
.	O
</s>
