[immibis]

If the PoW cost is a low-end cellphone CPU for 2.5 minutes, then it's nothing to the spammer with the 200-core hourly AWS server. If each spammer can create 10000 identities (not connections, identities) per hour, then you might as well not have a limit at all. If they could even create only 2 identities per day that would be enough to spam with (yet still unacceptable to actual users). 250000 identities per day is way too many.

[kragen]

The speed ratio is much smaller than you say with memory-hard PoW problems, which depend on the amount of RAM you have (and its response time). But it's surely true that a spammer could create many accounts per day, perhaps 1000 per hour on a big server, which could then go on to spam a few accounts each before becoming uneconomical to keep using.

But that would still put the CPM of the spam around US$2, which very few spammers can afford. Maybe mesothelioma lawyers and spearphishers.

You don't have to make spamming physically impossible, just unprofitable.

[immibis]

A single identity can send messages to hundreds or thousands of users.

[kragen]

You're talking about a different proposal than the one I wrote in the comment you were replying to, then.