I don't need this feature myself, just thought it might be an idea for a product feature.

I base it on personal experience of having been been called in to give advice on several cases of excessive spam. In almost all such cases the problem originated from numerous unprotected mailto: links on associated webpages. These are harvested by robots, and the addresses added to "email marketing" lists. Typically the spamming will continue for several weeks after a single harvesting incident, and then taper off. However, if repeated harvesting of the same email address from multiple webpages occurs, then the volume of spam will begin to ramp-up to levels where it is a major problem.

Many small-site admins are completely unaware of address harvesting, and will blame the mailserver software for a problem which actually lies elsewhere. This damages the mailserver product's reputation, and gives rise to a poor user experience.

I agree that it would be better to tackle this (extremely common) issue on the webserver side of things, but there you are up against tremendous inertia in trying to get commercial website maintainers to change their bad habits. A mailserver feature which at least warns of the reason for a flood of spam would be helpful to most sites.