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Linux as an email hub.

Posted on 1997-04-22
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Last Modified: 2010-03-18
Linux is bouncing incoming email between it and the email relay.  The email relay forwards the mail correctly but when it gets to the Linux box. Sendmail does know that it is suppose to keep the mail and give it to the appropriate user on the Linux box.  The email relay and Linux email hub are on seperate networks.  How do we get sendmail to keep the mail?

thanks

-brandon
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Question by:blnewpor
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northwind earned 100 total points
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Informations are not fully sufficient, but I will try anyway.

That you have the relay, seems to indicate that you know how the MX-records works - and yet it is here that the key to the problem lies.

Usually the situation is that you have an internal "unreachable" network like 192.168.x.0 (firewall scenario) and your MX points to the firewall. (Mine does that)

So the mail goes there.

At the firewall, the firewall discovers that the mail goes internally. This ccan be achieved in different ways.

So the mail goes there.

Now why would it bounce back? Well there iss actually only two reasons.
1) You have full nameservice here. So the SMTP MTA (Mail Transport Agent) sees your MX and says "hey it should actually go elsewhere".
The solution is of cause to keep the internal resolver unknowing of that.
Set up the nameservice on that machine so that it can not get access to the domain information (especially the MX) from outside.

2) The machine is not who he says he is. You told the firewall that olsen.com was on the internal net, but olsen.com has in reality another name. The mail goes there, but the MTA recognizes it as being for somebody else.
Solution: rename the machine or tell the MTA that it masquerades for this machine.

Most likely it is 1)
I maintain the yarc.com domain besides others. Mail to yarc.com is MX'ed to janus.yarcrip.com.
Janus.yarcrip.com has a broken nameservice. It knows that yarc.com is on the internal network - and where.
So it send the mail there.
Yarc.com on the internal net has a broken nameservice as well. I told it that it maintains primary nameservice for yarc.com (which it don't) and that it itself is yarc.com. Thats how I get rid of the MX on the internal network.
The nameserver here knows that all other namelookups must be forwarded to the outside nameserver, making this machine perfectly capable of handling all in and outgoing mail.

Hope that gets you going
Karsten Jeppesen <kj@yarc.com, northwind@earthlink.net>
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