DNS is a naming system which translates Domain Names (web addresses meaningful to humans) into numeric IP Addresses (meaningful to ISPs). In simple terms, the Domain Name System serves as the "internet phone book" by translating human-friendly Domain Names into IP Addresses. For example, www.customerminds.com translates to the IP Address 184.108.40.206.
To detect and prevent attempts by spammers, ISPs perform both forward and reverse DNS look ups. A forward DNS lookup will first check the sender’s Domain Name, and then verify the associated IP Address. A reverse DNS lookup will first check the IP Address and will then verify the registered Domain associated with it. With either a forward or reverse DNS look up, any inconsistency between the IP Address and the Domain Name, the email could be classified as spam. Ensuring that your DNS Settings and SPF Records are correct helps ISPs to verify your identity and ensure maximised deliverability.