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Beginners Question

Posted on 2001-07-14
Medium Priority
Last Modified: 2013-12-17
I have Sendmail on my system but cannot seem to do anything.

Whenever I try to send an email from the machine I get a permanent fatal error in trying to resolve the mail address even though I iknow it is correct.

Any ideas of where I should be looking to solve the problem.

Thanks for the help.
Question by:RICHARDH
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LVL 40

Expert Comment

ID: 6282144
That could be a result of several different problems. For sendmail to work correctly the following conditions need to be met.

The hostname of the SMTP server must be set to what a reverse lookup of its Internet link IP yields. If that's not done a number of the Internet mail servers will refuse to talk to your sendmail server.

DNS must be correctly configured on the system so that sendmail can resolve external hostnames. If your Internet link isn't usable when sendmail first starts, you'll have to have a record in /etc/hosts for the system. Otherwise sendmail won't start up properly as it won't be able to do a reverse lookup to determine its IP.

There are other things that can be a problem that are outside of your control. Your ISP may have filters in place that prevent their clients from passing SMTP traffic to anywhere except the ISP's mail server. Or the IP address range used by your ISP may be listed in the Dialup User List (see Both of those measures are a means of limiting the ability of an ISP's clients to generate spam.

To fully participate in Internet mail using only your system you need a properly registered domain name and the DNS records that go with it. That also implies that you have a full-time Internet connection and that your ISP allows their clients to run their own mail servers. I'd advise you to check the Terms And Conditions of your user agreement to make sure that you aren't going to violate the agreement. Some ISP's are very militant about that and have been known to terminate service to users that don't abide by the agreement.

Author Comment

ID: 6282273
Thanks for the advice. I do have a permanent connection to the internet and the ISP does allow mail to be sent, the problem is probably that I do not have a unique Domain name to use on the machine (Do I need that).

The situation is that we are running a mail server on x.x.x.3 and I want to run this one on x.x.x.4. Is it possible to do that with the same domian name.

Thanks for the help. It is most appreciated.

LVL 40

Expert Comment

ID: 6282357
Okay, so we aren't going to run afoul of ISP restrictions and since you already have an SMTP server running from the local LAN most of the other possible restrictions don't apply.

There isn't any problem in having more than one sendmail server running within a domain. The only gottcha is that the SMTP server that holds the MX record for the domain will receive all email addressed to user@your-domain.tld. You can set up an alias or virtusertable entry on that SMTP server to cause inbound email to be redirected to another mail server. If the "Reply-To:" on all outgoing messages is set to user@this-system.your-domain.tld returned mail will come directly back to your system, provided the DNS is correct.

Is there a registered domain name for this network? This system will need a hostname entry in the DNS for its IP and it will have to have that hostname assigned to it. Also you need to make sure that this system can resolve Internet names, i.e., you've got the correct data in /etc/resolv.conf. You can check to see if DNS is correctly configured by attempting to connect to some web site or by executing an nslookup of some hostname, e.g., "nslookup".
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Author Comment

ID: 6284976
Thanks for the help so far.

I have a spare domain name that I will get assigned to the machine I am using. This will take a couple of days.

When I have done this, perhaps I can get in touch again and you can give me some more help. That would be very much appreciated.



Author Comment

ID: 6297667
OK, I am back again.

I now have the machine set up with a Domain name. It is woprking OK as I can get to the server using However, I still cannot send any emails from the system, it just gives me fatal errors with every address I try to use.

Any suggestions as to what I should try.


Richard H
LVL 40

Expert Comment

ID: 6298456
Right... We need to find out if the server is properly resolving via DNS. To do that go to the server and try "nslookup" and let me know what happens.

BTW which OS is the server running?

Author Comment

ID: 6298767

I did nslookup as you suggested and it came back with two IP addresses.

Thanks for the help

Author Comment

ID: 6301297
Sorry forgot the last part of your question.

I am running the latest version of Redhat Linux. I also use something called Webmin to administer all the servers, very useful.

Thanks again for the help
LVL 40

Accepted Solution

jlevie earned 200 total points
ID: 6303969
Ahh, webmin huh? Have you attempted to use webmin or linuxconf to configure sendmail on that system? In my experience both are likely to muck up the file which can easily prevent sendmail from working.

On the assumption that by latest version of RedHat you mean 7.1, then the default file won't be very usable as is. For security considerations sendmail on 7.1 is shipped such that it will not accept inbound connections. That's very easily fixed by chommenting out the line in /etc/mail/ that reads:

DAEMON_OPTIONS(`Port=smtp,Addr=, Name=MTA')

and build & install a new with:

# cd /etc/mail
# m4 /etc/mail/ > /etc/

At least part of the DNS config on this box is correct in that nslookup can resolve DNS names. The next question is whether the system is using DNS as the defaul name service. From the install of RedHat 7.1 /etc/nsswitch sets the order of nostname resolution like:

hosts:      files nisplus nis dns

Whis is fine as local name services will be check first (if they exist in the case of nisplus & nis) and finally dns will be checked. See if your system is set up in this way and it would also be good to be sure that something that indirectly uses DNS for hostname resolution works. That's easy to do, assuming you aren't using a local proxy server, by usng Netscape or Mozilla to look at one or more Internet sites. If you have local proxies for Web & FTP you can attempt to open a telnet session to some Internet SMTP server, like:

chaos> telnet 25
Connected to
Escape character is '^]'.
220 ESMTP Sendmail 8.11.0/8.8.7; Fri, 20 Jul 2001 19:02:55 -0400QUIT
221 2.0.0 closing connection
Connection closed by foreign host.

Expert Comment

ID: 6782310

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Author Comment

ID: 6804213
Am tidying up my site and find that I had not awrded you the points.

Thanks for the help and apologies for the delay.

Richard. H

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