UNIX hosts and MX "best practice"

[Barry Margolin]

In article <23058.949106728 at gromit.rfc1035.com>,
Jim Reid  <jim at rfc1035.com> wrote:
>A bigger issue is the impact on the mail delivery and queues when mail
>is sent to hostnames that don't have MX records. Suppose that mail is
>sent to user at foo.example.com and there's no MX for foo.example.com.
>The A record is looked up and the mail system will try to connect to
>port 25 on foo.example.com and speak SMTP. If foo.example.com doesn't
>have an SMTP listener, that connection will not succeed. The mail will
>get queued and sendmail (or whatever) will keep trying to deliver it
>for a few days or so before giving up. The mail system can't tell the
>difference between a remote system that's temporarily switched off its
>SMTP listener and one that *never* has such a thing. So the mail
>system's queues get cluttered with undeliverable mail and the
>postmaster gets a headache. Meanwhile the user is unhappy because
>their mail hasn't been received. Maybe the user was wrong to send mail
>to foo.example.com when it didn't have an SMTP listener. But if this
>host had an MX record, its mail could be delivered somewhere, even if
>it was only to some mail system that bounced the message because the
>email address was wrong.

I assumed we were only discussing whether a host should have an MX record
pointing to itself, versus just depending on the A record.  If its mail is
supposed to be delivered somewhere else, it's intuitively obvious that it
needs an MX record to get it there.  Why would you have people send mail to
user at foo.example.com if it doesn't run a mail server and there's no MX
record redirecting its mail?

-- 
Barry Margolin, barmar at bbnplanet.com
GTE Internetworking, Powered by BBN, Burlington, MA
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