1. A computer-implemented method comprising: receiving a first query; receiving, from a search engine, data identifying first search results that the search engine identifies as responsive to the first query, wherein each of the first search results has a respective ranking score; determining that at least a first threshold number N of top-ranked first search results reference resources that are located on sites that are previously identified as low-quality sites; in response to determining that at least the first threshold number N of top-ranked first search results reference resources that are located on sites that are previously identified as low-quality sites, obtaining a second query from a predetermined mapping of queries to alternative queries; receiving, from the search engine, data identifying second search results that the search engine identifies as responsive to the second query, wherein each of the second search results has a respective ranking score; determining that at least a second threshold number M of top-ranked second search results reference resources that are located on sites that are previously identified as high-quality sites; and in response to determining that at least the second threshold number M of top-ranked second search results reference resources that are located on sites that are previously identified as high-quality sites, providing one or more of the second search results in response to the first query.