Meng wrote:
if people want to avoid the mandatory callback, they can use
a public-key scheme rather than a single signing secret.
If we define "callback" as "upon receiving an email, contact the sender and
retrieve some information from it", then even with public keys a "callback"
is mandatory, namely to get the public key.
Of course you could cache the public keys. But then you would have to add a
cache management (with TTL and so on) to your MTA. And this would require
several megabytes of memory.
then a receiver could download the SES pubkey and verify
localparts without the callback.
Doesn't this use quite some CPU resources which the receiver has to pay?
Better would be if the sender (spammer) had to do the heavy calculations.
Roger