Sendmail cannot send external email

I have setup sendmail to use port 26 as smtp, I have no problem sending internal email but when I send external email eg. yahoo mail, it keep showing the following  errors

Deferred: Connection timed out with mx4.mail.yahoo.com.
Message could not be delivered for 5 days
Message will be deleted from queue

I have no problem receiving external email because I use a mail port redirection from port 25 to port 26 as my ISP had block port 25

Any idea how to resolve this problem?

ghganAsked:
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wesly_chenCommented:
> my ISP had block port 25
-- http://help.yahoo.com/help/us/mail/pop/pop-06.html --
If your ISP blocks port 25 or if you're unable to send email, then you will need to use port 587 when sending via Yahoo!'s SMTP server. To make this change, please follow the directions below:

From the "Tools" menu, select "Accounts"
Select your Yahoo! POP account and click on the "Properties" button
Click on the "Advanced" tab
Next to "Outgoing server (SMTP), change port 25 to 587
Click "Apply", then click "OK" and "Close"
----
Wesly
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jlevieCommented:
When your ISP blocks port 25 they only do so for mail that does not flow through their mail server. This means that the solution to your problem is to configure Sendmail to use your ISP's mail server as a smart host for outbound email. This can be accomplished by including:

define(`SMART_HOST',`mail.my-isp.net')

in the mc file used to build sendmail.cf and run Sendmail on the standard SMTP (25/TCP) port. This will solve the problem of outgoing mail. For inbound mail configure Sendmail to listen on 587 in addition to port 25 and configure mail redirection for 25->587. Or have accounts for each user on your ISP's mail server and use fetchmail.
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ghganAuthor Commented:
Hi jlevie

That is not an option, the reason being my users might not be a subscriber of the same ISP, furthermore I have enable smtp-auth. Anyway I have try telnet to port 26 at ip address 202.156.148.198 and it does not encounter any problem and I am using Fedora 1
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jlevieCommented:
In order to exchange mail with other MTA's your server must be using a standard SMTP port since the MTA's on the Internet are only listening for connections on those ports. By and large that means 25/TCP. And in your ISP blocks that port for outbound(and possibly inbound connections) you will have to use your ISP as an outbound relay, which in turn means that your Sendmail must be using the standard SMTP port.

In you configure Sendmail to listen on both 25 and 587 you can relay outbound via your ISP's mail server and change your mail redirection to be 25->587 so that inbound mail comes through. That also means that your clients will need to use 587. The standard sendmail.mc on Fedora contains commented out directives for enabling 587 as an additional listening port.
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makhanCommented:
Just a thought

>> That is not an option, the reason being my users might not be a subscriber of the same ISP, furthermore I have enable
>> smtp-auth.

You could perhaps use `RELAY_BASED_ON_MX' feature.
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jlevieCommented:
My pardons, but... That still misses the point that to send mail to any external MTA Sendmail has to be using 25/TCP since that's all the remote mail servers are listening on. And that is the cause of his failures since this Sendmail is using 26/TCP to avoid the ISP blocking on 25/TCP.
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makhanCommented:
I agree to jlevie.
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ghganAuthor Commented:
I am wondering, is it possible to use iptables to redirect my smtp port 26 to port 25 in the ethernet outgoing interface. If so, will it resolve my problem since I found out my ISP only block incoming TCP 25.
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jlevieCommented:
You might be able to make that work, but it would seem simpler to me to set up Sendmail for the default SMTP port and to enable the alternate SMTP port (587/TCP). Then change the 25-26 mapping to be 25-587.
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