[jchw]

Why per address, you ask? My opinion on why is because it is a finite namespace, so if any single person took a disproportionate amount of names it would adversely impact others. You can approach that by assigning a cost to addresses and adjusting as necessary and/or using quotas to limit abuse/over use of the namespace. In addition, an email provider’s reputation hinges on its ability to prevent abuse on its own platform; the ability to acquire huge numbers of addresses or rapidly switch between addresses would effectively make the provider a safe haven for spammers who are difficult to block externally without blocking the whole provider.

This is mostly conjecture, though; I don’t actually know for sure how it’s viewed by providers and I also don’t have a complete view of how SPAM prevention actually works (certainly seems complicated.)