This was a question from a college in Colorado. They had a student email account hijacked from a phishing scam and it was then used for a mass outbound spam attack. The question became: How I can prevent this? Is there a way to throttle outbound messaging per user? The answer, coming from Program Manager Todd Luttinen, is yes.
In Exchange 2010, there is a new cmdlet called New-ThrottlePolicy within that cmdlet you can set the RecipientRateLimit which limits the number of recipients a user can address in a 24-hour period. Having a low 24 hour send limit is the method for throttle mass spaming outbound per user. I believe Outlook Live has this outbound spam protection value set to 250 users per day per mailbox.
Read more about the cmdlet here.