Complex sentence: A server should correctly validate DNS responses to ensure that they are from an authoritative source (for example by using DNSSEC); otherwise the server might end up caching the incorrect entries locally and serve them to other users that make the same request.
Split into simpler sentences: A server should correctly validate DNS responses to ensure that they are from an authoritative source (for example by using DNSSEC). Otherwise the server might end up caching the incorrect entries locally and serve them to other users that make the same request.