This is my list of Banned Email Domains on my wpmu.

You can use to protect against spammers with another protection tools. You cat put this domain in your WPMU admin on Option -> Banned Email Domains.

.co.cc
2424blog.com
900-numbers-livetoday.co.cc
azbestsellers.com
buffalomission.org
conficker-worm-removal.com
d6327.com
dazzlepedia.org
danielamail.com
dentistinnewportbeach.com
discoverrome.org
ddoforums.com
emaildesechable.info
emailfile.info
emailftw.info
fatbookmarks.com
filmstarwallpapers.com
free-drumlessons.com
freebassguitarlessons.net
freeeeemail.com
googlemails.net
hopandshop.co.uk
ihonn.net
internetsalestactition.com
iwantmail.org
lung-research.com
medialeaks.com
mexicanemail.info
mirroraty.org
mypainbuster.com
my-secret-tips.com
mysace.info
onlq.com
partyopedia.com
slrreview.net
solarhotwaterheater.eu
solutionsinnovative.com
thailandtravelview.com
uricacids.com
yargies.com
waystoquitsmoking.eu
williamemail.com