Greylisting is one of those techniques I always loved, as it was so simple and logical.

As you say these days spammers will retry, but its an approach that still has value, because in the period between the initial delivery-attempt (which is rejected) and the final one (where it would be attempted) it is more likely that the sender has already appeared on DNS-based blocklists.

But there is also the problem that some people don’t want to wait multiple minutes or hours for an email, like password reset emails. I see too many disadvantages.