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Re: Email System Model

2009-05-21 10:32:06

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?

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