When a host searches for a domain, it repeatedly scans the entire DNS hierarchy until it finds the specific domain. An Internet service provider (ISP) provides recursive servers that perform these searches. When this server performs an IP address lookup, the server sends the request to the root servers. It will then choose one capable of providing a response. If that doesn’t happen, it will choose another server from the list, and so on. They may check several servers before finding the one that matches the request. In the case of a reverse DNS resolution, i.e. a lookup using your IP address first, the search is done through a PTR input. This indicates the server host name that matches the relevant server address. If specified, it must contain the reverse record of a DNS A or AAAA record. Requests from online mail servers (emails, etc.) do not have a PTR record, so they are more likely to fail.