port question

[David A. Bandel]

On Sun, Aug 23, 2009 at 12:20, Tony Alfrey<tonyalfrey at earthlink.net> wrote:
> Since we're on this topic perhaps someone can confirm/deny my conclusions:
>
> 1.  An SMTP *client* within a mail client such as Thunderbird, sends
> outgoing mail to an SMTP *server*.  Correct?

outgoing mail uses SMTP, yes.  incoming uses either imap or pop

>
> 2.  That SMTP *server* may then send mail to a receiving SMTP server (via
> the internet or other network).  Correct?

yes.

>
> 3.  The SMTP *client* in (1) sets a port within the TCP/IP protocol packets
> of the outgoing mail.  If Port 25 is set, it may be rejected by the outgoing
> SMTP *server*, and never re-sent (relayed).  Correct?

the smtp client may request a different port, but that would usually
be because the network the client is on has port 25 blocked at its
border and the smtp server is beyond the border

>
> 4.  The receiving SMTP server may also possibly reject Port 25 if sent by
> the transmitting SMTP server.  Correct?

Not likely, at least, not if it is a real mail server.  What it might
do is demand SMTP auth (authentication by the user) if the recipient
of the e-mail is not local (i.e., the smtp server is being asked to
relay).

>
> 5.  An ISP used by a mail client to reach an SMTP server may reject Port 25
> packets.  These packets may never even be seen by the first SMTP server
> intended for sending the e-mail.  Correct?

No, see the previous explanation.

>
> In other words, there are several places where Port 25 may be blocked by
> many SMTP servers or ISP that may lie between a user e-mail client on a home
> machine (an SMTP client) and an eventual SMTP server used to receive, and
> store, email for the intended recipient.  Correct?

Outgoing port 25 may be blocked at the border of a network, but only
for clients, never for the ISPs mail server.  This prevents Windoze
spambots from polluting the rest of the internet -- they must
authenticate to an ISPs mail server which will then relay the mail out
(using port 25).


David A. Bandel
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