I'd like to think of myself as an intermediate-level Linux user. I'm trying to set up sendmail to be just a relay server, but can't really find any information related to what I want to do...

The long and short of it is this: I'd like to set up sendmail as a backup mail server to my domain, which sits on a different ISP Connection, and I wish it to accept messages only for my domain name, and hold those messages until my primary mail server comes back online, then deliver the queued message(s) to the primary mail server.

If someone sends a message to, for instance, megabyte@mydomain.com, and the primary mail server for mydomain.com is down, then I want the mail to be sent to my linux machine running sendmail. The linux machine running sendmail would then just hang on to that e-mail, and periodically check to see if the primary mail server for mydomain.com is up. Once it finds that the primary mail server is up, it then delivers that message that it has queued to the primary mail server.

I don't wish this linux machine to do anything but that. No routing of mail for anyone, no mail delivery (other than delivering queued messages to the primary mail server), and no acceptance of any mail for any domain other than mail for mydomain.com.

Setting up the DNS is not a problem. This machine will be a standalone machine sitting in a datacenter. DNS will be somewhere else (and properly set up). All this machine will do is serve web pages (as a backup to my primary connection) and queue mail (again, as a backup).

Can anyone give me a direction as to where to start?

Thanks

-Megabyte

Start here:
http://www.oreilly.com/catalog/sendmail2/

Make the linux machine the secondary MX and then add the domain to the relay-domains file.

Thank you,

The end result was very similar... It seems that sendmail.org was very helpful in this regard. Though most of the FAQ's and info that I found on the internet had me rewriting the sendmail.cf to make any kind of changes, I found that I didn't have to do any of that to get what I wanted to get done accomplished.

From what I found (and have tested and it works), the only thing I had to do was add a few lines to two different files (what made it worse was that those files didn't even exist in a standard RedHat 7.0 Installation so I had to figure out what they were). Those files were:

/etc/mail/access - in which I added the line:
mydomain.com RELAY

/etc/mail/mailertable - in which I added the lines:
mydomain.com esmtp:realmailserver.mydomain.com
.mydomain.com esmtp:realmailserver.mydomain.com

From what I found out, the access file overrides the default rule of no relaying, and the second file gives a specific location as the only place to try to deliver this queued mail - which was needed to resolve the issue that popped up - Error "553 - Config Error: Mail loops back to me (MX Problem?)"

From what I understand (and is, of course, possibly wrong) the /etc/mail/relay-domains file is for defining who is allowed to relay through the mail server out to the internet (like a local LAN addresses list), which ended up being exactly the opposite of what I wanted.

The end result was that I wanted ANYone to be able to relay through this mail server (as it is intended to be a secondary queueing server), as long as it was going TO a specific domain. And this is what I got. Thank you again