It should be pretty clear that reused IP space is going to represent a
problem. There is no mechanism for "LIR/ISP notif[cation to] those
blocking the addresses that the addresses have changed hands." Even if
there were, this would be subject to potential gaming by spammers, such
as SWIP of a block to SpammerXCo, followed by an automatic unblock when
the "ISP" unSWIP's it and SWIP's it to "EmailBlasterB" - of course, the
same company.
How do we manage this into the future? IPv6 shows some promise in terms
of delegation of larger spaces, which could in turn suggest that reuse
policies that discourage rapid reuse would be a best practice. However,
that is more or less just acknowledging the status quo; networks are
likely to continue blocking for various reasons and for random periods.
A remote site being unable to communicate with us is not particularly
important except to the extent that it ends up distressing users here;
however, for larger sites, the blocked list could end up being
significant.
It seems like it *could* be useful to have a system to notify of network
delegation changes, but it also seems like if this was particularly
important to anyone, then someone would have found a trivial way to
implement at least a poor man's version of it. For example, record
the ASN of a blocked IP address and remove the block when the ASN
changes...
... JG