DNS passthrough on no explicit result?

[Vernon Schryver]

>                                         You have records which absolutely
> need to be public: SPF, MXs--mail won't work otherwise.

I hope I misunderstood the intended meaning or context of those words,
because their literal, context free meaning that SPF and MX records
are required by SMTP is wrong.

SPF might be considered required by unsolicited or semi-solicited
bulk mail senders to help large scale "free" mailbox providers gauge
the legitimacy of mail advertisements.  Otherwise SPF is *not*
required.  As proof consider both this message and the DCC mailing
lists (i.e. old school solicited bulk mail.)  In some cases SPF
harms SMTP delivery, especially when combined with DMARC.

Because I'm in neither the email advertising business nor the large
scale "free" mailbox businesses, the only unambiguous use I've found
for SPF records is to try to prevent mail.  I publish SPF RRs for some
domains that send no mail in order to reduce NDRs or "bounces" of
forged mail from bad SMTP servers (mail receivers) that fail to validate
SMTP Rcpt_To values during the SMTP transaction.


The case for MX records being required for SMTP is clear.  In the
absense of an explicit MX record, the standards require SMTP clients
(mail senders) to infer an implicit MX from derived A or AAAA records.


Vernon Schryver    vjs at rhyolite.com