Minor quibbles:
The space of IPv4 addresses is 4.2 billion bits wide.
Suggest:
The space of IP addresses is wide - 32 bits or 4.2 billion possible
addresses (more with IPv6)
One point you might want to mention:
Spammers may turn to registering hundreds of domains for their own use. In
this case, designated sender schemes can be used in conjunction with DNSBLs
to block all domain names that are served from the same DNS server.
Another possible point, if you want to use it:
Some domains may choose not to publish designated sender info, or publish
too wide a range. [Gray graphic with white spots can show a white stripe
all the way across]. Recipients can determine for themselves if this
policy is "too promiscuous" and either refuse the mail or downgrade/filter
it.
--
Greg Connor <gconnor(_at_)nekodojo(_dot_)org>