Re: ISP blocking smtp port and a way how to solve the issue

From: wonder (a_at_b.com)
Date: 08/10/04 

Date: Tue, 10 Aug 2004 22:42:25 +0800

+Alan Hicks+ wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
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>
> In comp.unix.bsd.freebsd.misc, wonder dared to utter,
>
>>Before open up port 1025 for smtp, I use tcpdump listenting to port 25.
>>Now, directly connected email client failed to send email thru port 25.
>>There is no traffic can be seen by tcpdump on port 25 at this moment.
>>Use nmap scan from the client machine shown that port 25 at the mail
>>server side is being filtered. Actually I don't need to use nmap to
>>check it up, because tcpdump at the mail server box can't see traffic
>>arrived at port 25 regardless whehter the firewall at the mail server is
>>blocking the traffic or not, tcpdump should /see/ the traffic anyway.
>
>
> Then your ISP is most definately blocking inbound connections to port
> 25. They may not be blocking outbound connections for what it's worth.
>
>
>>The problem is I need to open port 1025 for direct email connection from
>>external email client to send email, and port 25 for mail server receive
>>external email from other domains.
>
>
> Ok... this is a very confusing statement. Here's the way I read it:
>
> a) I have to run sendmail on a non-standard port to send e-mail to my
> mail server.
> b) but sendmail has to be running on port 25 to receive mail from
> clients that I don't have control over.
>
yes, point a is what I meant.
With point b, the clients are those from external domains like yahoo and
hotmail, not the clients that belong to my domain name.

>
>>If ISP is not blocking port 25, why direct connected external email
>>client can't send email thru port 25? why telnet to port 25 is failed??
>
>
> What do you mean by "direct connected external email client"? This is
> confusing. Is this a serial connection or something? Is it outside of
> your ISP's network, or just outside of your LAN?
>
Sorry, "direct connected external email client" means those external
email clients belong to my domain name. Execuse my poor english.

>
>>If port 25 is being filtered, why mail home mail server can receive
>>external email from different domain?? Unless ISP's mail server doing
>>some tricks on it. Then what is it?
>
>
> If you are running sendmail on port 25 and can receive mail, but cannot
> send mail, this is a standard problem. Many ISPs block outbound port 25
> traffic except through their mail server. sendmail has something called
> "smart host" which will allow you to route all outbound e-mail through
> the ISP's mail server. Other MTAs have similar functionality.
>
Thanks, should I assign my mail domain name to the "smart host"?
I think in postfix, this is called relayhost. I found that it is
currently pointing to nothing.

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