Microsoft, world leader in Internet security, will doubtless clean up its spamming act when it sees its AS 8075 is #1 for outbound spam in the U.S. for April 2012 in rankings from PSBL data, pushing the U.S. to #1 worldwide. Other rankings don't show Microsoft high, but does MSFT really want to show up in any of these rankings?
of top 10
These rankings that show Microsoft high are derived by SpamRankings.net from PSBL blocklist data. The April 2012 SpamRankings.net from CBL blocklist data do not show Microsoft in the top 10. Apparently PSBL's spam traps happened to be in the line of spam from Microsoft, while CBL's were not.
And of course Microsoft probably doesn't mean to be sending any of that spam. More likely botnets exploited a MSFT security vulnerability. Here's hoping they clean it up soon!