Re: Email System Model

ned+ietf-smtp(_at_)mrochek(_dot_)com wrote:
> > Externally administered backup MXes run into backscattering because
> > they don't maintain a copy of the users database.
> Some don't, many do.
Hm... would you expand on that, please? I browsed a few backup MX 
providers (DydDNS, ZoneEdit and Mailfail) and saw no evidence that 
they do.
> > To amend that status
> > of affairs implies that a user's email address will also be stored at 
> > an externally controlled backup MX. Such situation should interest the 
> > users, as those addresses are part of their personally identifiable 
> > information.
> Not necessarily. The obvious trick is to store hashes of valid addresses 
> along 
> with per-domain rules for how to strip stuff like subaddresses, prefix 
> characters, etc. Yes, a dictionary attack can be performed on this 
> information, 
> but you can also do that by whacking on the primary MX.
More easily, a backup MX provider can just store each hit it finds. 
However, even if cute tricks may rule out any relevant information 
leakage, in principle, users should be aware of what organizations 
take part in managing their data. Currently, that info is relegated to 
a non-machine readable ISP's policy page, if any.
The rule to strip subaddresses is a good point. Apparently, a regex 
might suffice, although in some cases passing the complete subaddress 
(or its hash) may be preferable. What about honeypots?