port question

[Bill Campbell]

On Sun, Aug 23, 2009, Bruce Marshall wrote:
>On Sunday 23 August 2009, Tony Alfrey wrote:
>> I must select Port 587 for the SMTP server settings when I use my e-mail
>> client (SMTP client?), otherwise I cannot get my packets to be accepted
>> by my SMTP server (earthlink) or allowed for transmission to my SMTP
>> server by my isp (comcast).   But you (and other) sources say that Port
>> 25 is always used for internet e-mail.  Does this mean that the port ID
>> in the packets is changed from port 587 to port 25 (or additional port
>> 25 headers added) by the transmitting SMTP server?
>
>Let's talk in big generalties here....    Port 25 is the common SMTP port and 
>is almost always used. to RECV mail at an ISP or in some cases and end user 
>running an SMPT server.
>
>Yes, a lot of ISP's will use port 587 (dunno why other than to cut down on 
>people trying to break in on 25) for their SMTP servers but they will usually 
>also require some means of authentication...   to avoid spammers from using 
>their server.

Port 587 is for SMTP-AUTH, requiring authentication, and probably
TLS/SSL encryption, not for SMTP without authentication.

To add confusion, port 465 is listed in /etc/services as smtps
which I presume is smtp with SSL although it may also be used as
standard SMTP as a way to work around broadband providers that
block port 25 incoming to non-commercial accounts.

Generally speaking, if you want to run a mail server receiving
mail on a residential broadband connection (a) you are probably
violating the contract which forbids servers, and (b) it helps to
have a friendly site that will act as you MX server receiving
mail from the Internet, and forwarding to your server by some
port other that 25 or via another transport such as uucp over tcp.

Some ISPs/broadband providers also restrict outgoing port 25
traffic to their own servers, and may restrict the Sender/From
addresses to ones in their domain.  It this case it really helps
to have a co-operative site that you can connect to on some port
other than 25 that the ISP is not trapping if you want to send
with other domains in the Sender/From address.  Think of this as
a smart-host that will accept all mail from your system, and
relay it to the Internet.

Bill
-- 
INTERNET:   bill at celestial.com  Bill Campbell; Celestial Software LLC
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