I am confured about this problem: for example, I request that a SMTP
server deliver a email with recipient is "xxx@yahoo.com", what happens
next in the SMTP server? Will it connect to the SMTP server of Yahoo.
But how can it know the address and port of the Yahoo server?

And in programms like Mail Direct Pro, which is a local SMTP server,
they will fail when the ISP blocks the outgoing port 25. Does that
mean that these software use the port 25 for the delivery of email to
the destination SMTP server? But Yahoo actually uses port 465 for its
SMTP server?

Thank you very much!

In article <1187397613.618836.92670@a39g2000hsc.googlegroups. com>,
"lichuan.nj@gmail.com" <lichuan.nj@gmail.com> wrote:

> I am confured about this problem: for example, I request that a SMTP
> server deliver a email with recipient is "xxx@yahoo.com", what happens
> next in the SMTP server? Will it connect to the SMTP server of Yahoo.
> But how can it know the address and port of the Yahoo server?
>
> And in programms like Mail Direct Pro, which is a local SMTP server,
> they will fail when the ISP blocks the outgoing port 25. Does that
> mean that these software use the port 25 for the delivery of email to
> the destination SMTP server? But Yahoo actually uses port 465 for its
> SMTP server?
>
> Thank you very much!

Initial mail submission and mail transport between SMTP servers are two
slightly different things.

Yahoo's use of TCP port 465 is strictly for initial mail submission from
their users. That port is commonly used for SMTP wrapped in SSL
encryption, technically a non-standard application but one that became
popular because Microsoft chose that port for their implementation.
Encryption at the transport layer is important at that phase because
typically initial submission includes some form of authentication in the
SMTP layer, and wrapping the entire session in SSL prevents snooping on
that authentication.

The standard TCP port for mail transport using SMTP is 25. There is a
standard SMTP extension (STARTTLS) to allow post-connection negotiation
of encryption on SMTP sessions, and a combination of a subset of
SMTP plus security extensions defined for use on port 587 for mail
submission.

--
Now where did I hide that website...

In article <1187397613.618836.92670@a39g2000hsc.googlegroups. com>,
"lichuan.nj@gmail.com" <lichuan.nj@gmail.com> wrote:

> I am confured about this problem: for example, I request that a SMTP
> server deliver a email with recipient is "xxx@yahoo.com", what happens
> next in the SMTP server? Will it connect to the SMTP server of Yahoo.
> But how can it know the address and port of the Yahoo server?
>
> And in programms like Mail Direct Pro, which is a local SMTP server,
> they will fail when the ISP blocks the outgoing port 25. Does that
> mean that these software use the port 25 for the delivery of email to
> the destination SMTP server? But Yahoo actually uses port 465 for its
> SMTP server?
>
> Thank you very much!

Initial mail submission and mail transport between SMTP servers are two
slightly different things.

Yahoo's use of TCP port 465 is strictly for initial mail submission from
their users. That port is commonly used for SMTP wrapped in SSL
encryption, technically a non-standard application but one that became
popular because Microsoft chose that port for their implementation.
Encryption at the transport layer is important at that phase because
typically initial submission includes some form of authentication in the
SMTP layer, and wrapping the entire session in SSL prevents snooping on
that authentication.

The standard TCP port for mail transport using SMTP is 25. There is a
standard SMTP extension (STARTTLS) to allow post-connection negotiation
of encryption on SMTP sessions, and a combination of a subset of
SMTP plus security extensions defined for use on port 587 for mail
submission.

--
Now where did I hide that website...