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

From: +Alan Hicks+ (alan_at_lizella.netWORK)
Date: 08/10/04 

Date: 10 Aug 2004 08:51:54 -0500

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

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.

> 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?

> 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.

- --
It is better to hear the rebuke of the wise,
Than for a man to hear the song of fools.
Ecclesiastes 7:5
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.2.1 (GNU/Linux)

iD8DBQFBGNKclKR45I6cfKARAu/KAJ4uTk679es7fuDnktIVBiXy8aHaWwCglg8M
yBQHyBuH/1m/GxcvxFPQW7I=
=be9M
-----END PGP SIGNATURE-----

Relevant Pages

  • Port 25 closing?
    ... connections to our mail server via port 25. ... port 25 (it can still RECEIVE e-mail since that goes over secure IMAP ... I can log onto the firewall, ...
    (Fedora)
  • Re: ISP blocking smtp port and a way how to solve the issue
    ... directly connected email client failed to send email thru port 25. ... >>arrived at port 25 regardless whehter the firewall at the mail server is ... They may not be blocking outbound connections for what it's worth. ...
    (comp.unix.bsd.freebsd.misc)
  • Re: OT .. Road Warrior communications question
    ... The data on the Internet is sent in little packets. ... The packets addressed to port 80 ... Likewise, at the mail server receiving the packets, it knows the return ... Why would e-mail work on the web but not from your e-mail software? ...
    (alt.guitar.bass)
  • Re: iptables, DNAT, and SMTP
    ... Did you sniff traffic on the client, the mail server and the barracuda to watch what's going on? ... connections to xxx.184.64.3 act as if there is nothing ... When nothing listens on a TCP port, ... Actually the SNAT rule is just an easy way to make the barracuda's replies go back to the NATing box without using special routing tricks. ...
    (comp.os.linux.networking)
  • Re: E-mail problem
    ... Port 25 is the classic port for POP3 mail servers. ... It's just not being used to access the incoming mail server. ... When the ISP requires secure password authentication, rather than clear text authentication, to it's service account, it must use the new port assigned for secure password authentication. ... It is designed to be used when you are outsdide the firewall. ...
    (rec.outdoors.rv-travel)