creating DNSBL for blocking email virus, need suggestion

[Markus <markus () jogja citra net id>]

Hello security-basics,

i'm inspired by virbl.bit.nl, and want to make DNSBL for my school
network to reduce the server load because of scanning all email with
virus that coming in.

VIRBL reduce the server load by creating an IP blacklist, if email
with virus sent from an IP then that IP will be added to the Blacklist
(BL), so the next email that sent from that IP will be rejected before
the antivirus scan it...

but i got some question..

how if the computer that infected by virus and send email virus is in
a network/LAN? the LAN's server public IP will be blocked, and then
all computer in that network can't send email to my school network.
can we know local IP 198.x.x.x for that computer?

how long do you think an IP should remain in the blacklist? because
blacklist can't know if the infected computer already cleaned by its
user/admin.

i appriciate any suggestion/idea..

thank you

-- 
Best regards,
Markus