Have you wonder how Microsoft IT defends against spam in our corporate network?

You might to check out this white paper "Messaging Hygiene at Microsoft: How Microsoft IT Defends Against Spam, Viruses, and E-Mail Attacks"

We employ a 3 phases approach to spam filtering as listed below and also maintain several best practices as mentioned in the link above.

Connection flitering - Block messaged based on regularly updated block list of IP addresses known to send spam

Sender/Receipient filtering -Block messages based on a block list created and maintained by Microsoft IT

Intelligent message filtering - Blocks messages containing content that is likely to be spam