I came across this comment on slashdot.org about Sender-ID to prevent spam:

If we let Microsoft, through some machinations during our anti-spam re-engineering or in any other manner, take any measure of control over what has, until now, been an 100% open-standard email infrastructure, email will be fragmented and ultimately ruined, far worse than any cadre of spammers could ruin it.

So borrowing from stpeter: