I have two Linux servers set up, and I am having a hard time getting them to send mail to each other. They are each set up behind the same firewall, but have a different domain name. What I want to do is be able to have them send mail to each other. Whenever I try, though, the receiving server doesn't seem to get anything, and I see the following in my log on the sending server:

Oct 5 22:11:36 zues sendmail[7858]: h92C5OBe010283: to=admin@mydomain
, delay=3+14:05:12, xdelay=00:00:00, mailer=esmtp, pri=7862515, relay=mydomain.com., dsn=4.0.0, stat=Deferred: Connection timed out with mydomain.com

The servers can "talk" to each other just fine otherwise. I am able to telnet and ftp between each of them no problem.

Is this a problem with Sendmail, DNS, or something else?

I am using Red Hat 9 with the most recent version of Sendmail.

do you have dns set up on any of the "local " boxes?
if not it is looking to an "outside" dns server that may or may not know about your
"internal" domains

an easier way to get around actually setting up DNS may be to use the /etc/hosts file

put entries in each one for each box. this way you dont have ot setup a DNS server. you cold also send by ipaddress:

To: root@[192.168.1.100]

that should also work.

I do have an entry in the /etc/hosts file on both my servers, so they can communicate with each other fine, such as with telnet and ftp. I can ping each server by domain name as well.

I do not have DNS set up on any of the local boxes.

When I try to send by ip address, I get the following error in my maillog:

Oct 6 06:26:05 zues sendmail[8380]: h96AQ56A008378: to=<root@192.168.0.5>, ctl
addr=<root@mydomain.com> (1068/1068), delay=00:00:00, xdelay=00:00:0
0, mailer=esmtp, pri=30348, relay=192.168.0.5, dsn=5.1.2, stat=Host unknown (Nam
e server: 192.168.0.5: host not found)
Oct 6 06:26:05 zues sendmail[8380]: h96AQ56A008378: h96AQ56A008380: DSN: Host u
nknown (Name server: 192.168.0.5: host not found)

This suggests maybe I do need to set up DNS on one of my local boxes:

Oct 6 06:27:05 zues sendmail[8380]: h96AQ56A008380: to=steve@mydomain.com
, delay=00:01:00, xdelay=00:01:00, mailer=esmtp, pri=31391, relay=otherserveralias.mydomain.com. [PUBLIC IP IS LISTED HERE], dsn=4.0.0, stat=Deferred: Connection timed out with
cs5.mydomain.com

I cannot ping either server with public ip, only domain name and local ip, which I think works because of my hosts file.

So.....is DNS must to send/recieve mail in a LAN between two PC...

Mairul: I'm not quite sure what your reply meant, but I am trying to send/recieve mail in a LAN between two PC's.

Do I need to have a DNS server set up on one of the local boxes for this to work?

What you're missing is an MX record. You *may* be able to trick sendmail by putting the alias "mailhost" (without the quotes) behind the IP + name of your other server in the /etc/hosts file, but I rather doubt it... (I think that will send *all* mail to the other server, and vis versa).

What you should do is setup a mini-internet (little 'i') with at least one DNS server and set each box to query it. You will need two domain zones (domain1, domain2) and each one will have a different MX . So at minimum you'll need:
1 DNS server
2 "forward" zones
1 "reverse" zone (not counting 0.0.127.in-addr.arpa)
1 "A record" for each zone
1 "MX record" for each zone

By the way, I should point out that this is the way to set it up if you want to simulate two sites on the Internet (for experience). It's possible to use Sendmail's mailer table maps to cause mail for either domain to be delivered to what ever host you want (with no DNS at all).

If you want to get Internet experience, use the setup with DNS. If you want to get LAN/WAN experience, use the mailer table setup.