Michael Deutschmann wrote:
> Once an honourable mail admin *knows* that a given message is
> a trusted forward, he must turn off spam defenses so that he
> doesn't force Problem B on an innocent other admin.
That would limit the next hop to the defenses available at the
forwarder, neither "better" nor "different" would be honourable.
That's unless defensive policies are shared within a given MRN. E.g.
(quasi) backup MXes may look up the same DNSBLs. However, such kind
of arrangements may conflict with the per-user varying nature of
MRNs, unless the union of all MRNs shares the same settings. Large
ISPs might impose that.
BTW, there is no authority who dares officially endorsing a DNSBL,
AFAIK. Once I've asked for that at a local ISOC meeting and they
laughed compassionately...
That may annoy the recipient, but it is unavoidable. Once the forwarder
has acknowledged the badguy's CR LF . CR LF it is too late to appeal the
forwarder's judgement. (unless the mail was SPF pass.)
A "smart" backup MX really should have some knowledge about what mailboxes
exist on the target, just like a forwarder does. In addition, the hosts
may exchange some transitional information (e.g. overquota) and cache it
for a few hours or until it is revised. Simple anti-spam recipes might
also be shared that way.
The recipient just has to learn to use the anti-spam "control panel" the
forwarder provides him, even if he finds it less user-friendly or capable
than the one at his home ISP.
For more involved content filtering that's unavoidable. Users won't get
the sum of all filters, but just the filter at the border. If they extend
the border, they must put there any required defenses as well.
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