Sander Holthaus wrote:
[...]
Those md5's should be done carefully, as a sender might rewrite MIME
boundaries on the fly, assigning random values as required by the RFC.
Do those need to change on every retry as well? E.g. are they
rewritten / parsed as the message is send?
They _may_ change, don't _need_ to. An MTA might store messages as
binary files and do MIME encoding on the fly based on the receiver
capabilities, althought that's not the most common behavior.
[...]
The remote IP address can also change, after load balancing or
more intricate sending arrangements. We could record the network
block, either after whois data or guessing.
Whois lookups would be pretty heavy, but some easy rules could apply
by as looking at the netblock or domainname. The domain-part is
unlikely to change and courier should have done a DNS-lookup so all
needed information should already be available.
SPF checking does much of that job. (Before DATA.)
Perhaps its result could be reused to make greylisting decisions.
In any case, it is better to think twice about it.
Routinely doing greylisting after DATA is a waste of bandwidth
that can become unacceptable (even if one may take comfort
thinking that spammers will have to pay that too...)
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