[ietf-dkim] Signalling DKIM support before DATA

[Douglas Otis]

On Aug 8, 2006, at 10:54 AM, Hallam-Baker, Phillip wrote:

> The way to do this (if it should be done) would be to use the SMTP  
> extension mechanism.
>
> For example. Define a command option SDATA, Receiver advertises it  
> in response to EHLO, sender uses it to present signed data in place  
> of DATA.
>
> Might be a better implementation but the point is we should use the  
> SMTP extension mechanism not invent our own.

Something a bit simpler that would not require changing the MTA  
code.  Use a host-name convention.  This would be apparent at the  
EHLO without requiring negotiations.  In essences,  
_dkim.mx01.example.com makes an assurance that:
  1) this host-name will authenticate
  2) a DKIM client policy can be applied

In conjunction with the DKIM client policy, there could also be  
MAIL_FROM policies that offer name relationships.  For example, this  
list might be a PTR RRset of names.  This might be a lookup of the  
(_DKIM-MP.<mail-from-domain>, PTR)
If a truncation occurs, a subsequent lookup of the (<query- 
domain>._DKIM-MP.<mail-from-domain>, PTR) extends this list  
indefinitely.

A domain name of "." might signal that the list is closed.

-Doug