<s>
SMTPS	B-Protocol
(	O
Simple	B-Protocol
Mail	I-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
Secure	O
)	O
is	O
a	O
method	O
for	O
securing	O
the	O
SMTP	B-Protocol
using	O
transport	B-Protocol
layer	I-Protocol
security	I-Protocol
.	O
</s>
<s>
SMTPS	B-Protocol
is	O
not	O
a	O
proprietary	O
protocol	O
and	O
not	O
an	O
extension	O
of	O
SMTP	B-Protocol
.	O
</s>
<s>
It	O
is	O
a	O
way	O
to	O
secure	B-Protocol
SMTP	I-Protocol
at	O
the	O
transport	O
layer	O
,	O
by	O
wrapping	O
SMTP	B-Protocol
inside	O
TLS	B-Protocol
.	O
</s>
<s>
Conceptually	O
,	O
it	O
is	O
similar	O
to	O
how	O
HTTPS	O
wraps	O
HTTP	O
inside	O
TLS	B-Protocol
.	O
</s>
<s>
This	O
means	O
that	O
the	O
client	O
and	O
server	O
speak	O
normal	O
SMTP	B-Protocol
at	O
the	O
application	B-Application
layer	I-Application
,	O
but	O
the	O
connection	O
is	O
secured	O
by	O
SSL	B-Protocol
or	O
TLS	B-Protocol
.	O
</s>
<s>
Since	O
whether	O
or	O
not	O
to	O
use	O
SSL	B-Protocol
or	O
TLS	B-Protocol
is	O
not	O
explicitly	O
negotiated	O
by	O
the	O
peers	O
,	O
services	O
that	O
speak	O
SMTPS	B-Protocol
are	O
usually	O
reachable	O
on	O
a	O
dedicated	O
port	O
of	O
their	O
own	O
.	O
</s>
<s>
"	O
smtps	B-Protocol
"	O
is	O
also	O
the	O
name	O
of	O
an	O
IANA-registered	O
service	O
,	O
with	O
the	O
TCP	O
port	O
number	O
465	O
.	O
</s>
<s>
The	O
service	O
was	O
intended	O
for	O
use	O
by	O
Mail	B-Protocol
Transfer	I-Protocol
Agents	I-Protocol
(	O
MTAs	O
)	O
,	O
as	O
a	O
point	O
of	O
contact	O
where	O
these	O
could	O
exchange	O
email	O
in	O
an	O
encrypted	O
form	O
rather	O
than	O
in	O
plaintext	O
.	O
</s>
<s>
When	O
describing	O
the	O
IANA	O
service	O
registration	O
,	O
the	O
official	O
capitalization	O
is	O
"	O
smtps	B-Protocol
"	O
.	O
</s>
<s>
When	O
describing	O
the	O
network	O
protocol	O
,	O
the	O
capitalization	O
"	O
SMTPS	B-Protocol
"	O
is	O
often	O
used	O
(	O
similar	O
to	O
how	O
HTTPS	O
is	O
capitalized	O
)	O
.	O
</s>
<s>
Port	B-Protocol
587	I-Protocol
is	O
the	O
well-known	O
port	O
for	O
submitting	O
mail	O
to	O
a	O
server	O
,	O
frequently	O
(	O
but	O
not	O
required	O
to	O
be	O
)	O
encrypted	O
using	O
STARTTLS	B-Protocol
.	O
</s>
<s>
Some	O
email	O
service	O
providers	O
allow	O
their	O
customers	O
to	O
use	O
the	O
SMTPS	B-Protocol
protocol	O
to	O
access	O
a	O
TLS-encrypted	O
version	O
of	O
the	O
"	O
submission	O
"	O
service	O
on	O
port	O
465	O
.	O
</s>
<s>
This	O
is	O
a	O
different	O
service	O
from	O
what	O
the	O
original	O
IANA	O
registration	O
dedicated	O
the	O
port	O
to	O
(	O
for	O
it	O
used	O
to	O
be	O
dedicated	O
to	O
encrypted	O
content	O
delivered	O
as-is	O
/	O
in	O
plain	O
text	O
,	O
whereas	O
nowadays	O
 '	O
SMTPS	B-Protocol
on	O
port	O
465	O
still	O
uses	O
plaintext	O
content	O
,	O
only	O
wrapped	O
within	O
TLS-encrypted	O
transportation-basically	O
the	O
reverse	O
mechanism	O
)	O
.	O
</s>
<s>
corrected	O
this	O
problem	O
(	O
a	O
special	O
exception	O
was	O
made	O
to	O
assign	O
TCP-465	O
to	O
two	O
use	O
cases	O
simultaneously	O
)	O
and	O
integrate	O
the	O
use	O
of	O
port	O
465	O
as	O
a	O
TLS-encrypted	O
"	O
submission	O
"	O
port	O
into	O
the	O
well-known	O
port	O
registrations	O
published	O
by	O
IANA	O
.	O
</s>
<s>
While	O
there	O
is	O
no	O
longer	O
any	O
officially	O
registered	O
endpoint	O
for	O
the	O
SMTP	B-Protocol
service	O
,	O
it	O
is	O
still	O
possible	O
to	O
exchange	O
email	O
over	O
an	O
encrypted	O
transport	O
with	O
similar	O
guarantees	O
as	O
those	O
offered	O
by	O
smtps	B-Protocol
,	O
in	O
particular	O
with	O
the	O
guarantee	O
that	O
either	O
the	O
exchange	O
succeeds	O
securely	O
,	O
or	O
does	O
not	O
happen	O
at	O
all	O
,	O
by	O
using	O
DANE	B-Protocol
in	O
combination	O
with	O
DNSSEC	B-Protocol
.	O
</s>
<s>
Many	O
email	B-Protocol
servers	I-Protocol
are	O
configured	O
to	O
either	O
not	O
deliver	O
email	O
securely	O
at	O
all	O
,	O
or	O
to	O
first	O
try	O
secure	O
delivery	O
with	O
the	O
STARTTLS	B-Protocol
mechanism	O
,	O
and	O
if	O
that	O
fails	O
,	O
for	O
example	O
,	O
because	O
the	O
remote	O
service	O
does	O
not	O
offer	O
it	O
,	O
or	O
because	O
a	O
successful	O
MITM-attack	O
has	O
stripped	O
announcement	O
of	O
the	O
feature	O
,	O
simply	O
fall	O
back	O
to	O
delivery	O
by	O
insecure	O
means	O
.	O
</s>
<s>
In	O
early	O
1997	O
,	O
the	O
Internet	O
Assigned	O
Numbers	O
Authority	O
registered	O
port	O
465	O
for	O
smtps	B-Protocol
.	O
</s>
<s>
Late	O
1998	O
this	O
was	O
revoked	O
when	O
STARTTLS	B-Protocol
was	O
standardized	O
.	O
</s>
<s>
With	O
STARTTLS	B-Protocol
,	O
the	O
same	O
port	O
can	O
be	O
used	O
with	O
or	O
without	O
TLS	B-Protocol
.	O
</s>
<s>
The	O
use	O
of	O
well-known	O
ports	O
for	O
mail	O
exchanges	O
communicating	O
with	O
SMTP	B-Protocol
was	O
discussed	O
in	O
particular	O
at	O
the	O
time	O
.	O
</s>
<s>
Port	O
465	O
currently	O
shows	O
as	O
registered	O
for	O
both	O
Source-Specific	B-Protocol
Multicast	I-Protocol
and	O
submissions	O
.	O
</s>
<s>
RFC	O
8314	O
"	O
Cleartext	O
Considered	O
Obsolete	O
:	O
Use	O
of	O
TLS	B-Protocol
for	O
Email	O
Submission	O
and	O
Access	O
"	O
proposed	O
to	O
recognize	O
port	O
465	O
for	O
implicitly	O
encrypted	O
email	O
submission	O
officially	O
.	O
</s>
