Blocking outbound port 25 blocks the vast majority of non authenticated smtp. The remainder being authenticated (or channelled via the same service provider, who can trace you by your IP, dynamic or not), provides some accountability and makes spam much easier to trace... acknowledging that much spam comes from compromised machines on residential grade connectivity (on port 25).

Blocking port 25 outbound (with an opt out option) makes sense if you can't quickly deal with offenders on your network (as often seems to be the case with big players). This doesnt the provide those players with an excuse to under-resource abuse@ (as the remaining spam is finding another way out) but this does seem to happen regardless... so its not even close to a silver bullet but it does helo more than hinder.

Mark.