E-mail,e-mail, bo-bee mail, banana fanna fo fee-mail--e-mail
Does your agency's Internet e-mail system play the name game correctly?
As the government's fascination with the World Wide Web builds, it's easy to forget
that most real business on the Internet is still conducted via e-mail. It's probably what
you miss the most when it goes down.
Yet many users ignore a basic warning sign that their mail systems are improperly
configured. If you sometimes get mail bounced back to you with the message "550 host
unknown," you may have a local problem, the system you're contacting may have a
problem, or both.
The Internet's Domain Name System translates every text address you enter into a
numeric address that's actually used to deliver your mail. For DNS to work, local name
servers on your network and others must be properly programmed and maintained so they can
map individual host names to IP addresses.
Local name servers usually are configured to know all local IP addresses, the host
names of the nine regional root zone servers that track addresses for all Internet
domains, and a cache of addresses from recent connections.
A local name server should have an internal name resolver that recognizes and routes
all incoming messages. Unless the local server is configured to recognize and pass DNS
mail exchange (MX) records, e-mail messages can't make it all the way through. This seems
basic, but it's ignored oftener than you'd think.
On the other hand, your local mail handler may indeed be configured correctly, but it
might receive, then reject mail that doesn't follow all Internet standards even though
there might be a way for it to accept the mail.
If someone sends mail to you that's bounced back with the "550 host unknown"
message, and you're sure your address was correct, ask the sender to try sending again
from the mail interface built into Netscape Communications Corp.'s Navigator 2.0 or a
higher version of the browser.
If the Netscape message gets through, it's because Netscape correctly looks for an MX
record first. The sender's regular e-mail system might have been set up to look first for
an address record, known as an A record, without ever attempting to find the MX record.
The A record defines the host-name-to-IP-address mappings, and there should be one A
record for every host and IP address combination. But using the A records for sending mail
is, well, sort of cheating. It's incorrect according to the Internet Engineering Task
Force's long-standing Request for Comments 974 and 1123 documents, which set the de facto
rules for this road.
But I've heard that some Unix operating systems still don't use MX, and some people do
use A records to route mail to you.
If you're having mail problems, the trick is to find a happy medium where your site
supports Internet standards for mail you send but also can accept mail that's not
Make sure your mail system uses MX records for outgoing mail. And take the time to play
with Unix sendmail, if you use that program, to see what you can do to correct problems
with incoming mail that doesn't follow the rules. You should be able to do it without
compromising your firewall.
Luckily, there's a wealth of information on line that can help you update your system.
To understand the Internet DNS system, check out the Web page at http://www.dns.net/dnsrd which has useful
pointers to DNS information.
For a handy glossary of DNS terms, visit http://castle.metainfo.com/MetaInfo/DNS/Glossary.htp,
and while you're there, look for the pointer to a good sendmail resources page.
To look up a DNS record for a site, visit http://burnet.del.t.u-tokyo.ac.jp/~nakai/resolver.html.
For e-mail related DNS questions, monitor the Usenet news group news:comp.protocols.tcp-ip.domains.
Not everyone will agree with this big-tent approach to accepting mail. One system
manager told me he's sick of trying to accommodate all mailers. "It's long since
passed the point where coddling non-MX-aware systems is worthwhile," he said.
"They either need to get fixed or hand off all outgoing mail to a system that does
use MX records."
Shawn P. McCarthy is a computer journalist, webmaster and Internet programmer for
GCN's parent, Cahners Publishing Co. E-mail him at firstname.lastname@example.org.