RE: Address to DNS mapping

> When we receive a MAIL FROM: (or HELO) command, it's argument
> is some random text string. Whether we do RMX, DRIP, SPF, all need to
> do a DNS lookup, and we all assumed to just cut the domain part from
> the address and use it as the place where to look at in the DNS.
>
> I don't think anymore that it is quite that easy:
>
> - We need to cope with empty sender addresses (coming with
>   error messages),

DMP addressed this using the HELO/EHLO identifier.  I think SPF adopted
this.  DMP introduced records for each FQDN authorized to send mail, so
you'd have records for hostname.example.com as well as example.com.  DMP
effectively treats the HELO/EHLO name as a mail domain ONLY for the purpose
of checking empty sender (null reverse path) senders.

This wouldn't prevent null sender spam from a FQDN but it'd clearly point
the finger at the owner.

> - sender addresses without domain

Must be local or authorized sender - either localhost, allowed IP or
authenticated IP such that delivery failures  can go to a mailbox on
localhost.

> - empty domain

Isn't this the same as sender address without domain?

> - malformed addresses (we need a syntax)

user@domain, RFC 2821.  Ignore bangpaths, Bitnet paths, etc.

DMP covered SMTP source routing a little, too.  If the receiver supports
source routing, check the first domain in the path, otherwise check the last
domain in the path.

> - Should we cut off the name part? Why not allow lookup of the
>   full address? And if so, how to avoid ambigousity?

We'd have to query the sender's mail server somehow to determine if the
username is valid or has a valid mailbox or alias.  I don't think admins are
going to put everyone's username in a DNS zone, even if it were closed to
zone transfer.  The whole privacy thing, after all.

> - What about encoding?

Of addresses in MAIL FROM or HELO/EHLO?  I think this is covered under
internationalization of DNS domains.  I forget the RFC.  RFC 2821 doesn't
refer to anything, I think it requires ANSI in HELO/EHLO and MAIL FROM.

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