North American Network Operators Group

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RE: mail does bounce (was: Customers down?)

  • From: Sean Donelan
  • Date: Sat Jan 01 22:46:08 2000

On Sat, 01 January 2000, "Roeland M.J. Meyer" wrote:
> > I agree with Greg Woods - if a domain is authoritatively non-existent,
> > I'd expect a sane mailer to bounce the message.
> I'm glad that you don't write code, especially sendmail code.


Would you rather be notified immediately if you mistype an address, or
wait four days before getting an undeliverable bounce.

If the name exists, but the mail host is unreachable try the next MX record
or if no more alternates, queue the mail.  If the name does not exist, return
mail to sender.  If you can't figure out whether the name exists or not, do

The RFCs just say a mailer is expected to do something "reasonable"
in reponse to a domain server error.  But RFC 974 doesn't specify what.
An otherwise compliant mailer can queue the mail or bounce it, and still
be considered compliant.

If a site wants to avoid this unspecified behaivor, a wise site would
try to assure availability of their DNS servers perhaps by using network
diversity so a single failure won't take down all its servers.

I remember setting up a connection last year with a major unnamed provider,
and pointed out a similar issue.  The other provider replied they had an
extremely reliable network, and there will never be any problem with their
DNS servers.  Less than a month later, the extremely reliable network barfed,
with the expected consequences.