We pick h.root-servers.net, one of the 13 root servers (1), and ask it about smtpr4.tom.com and learn that com is delegated to 13 name servers (2), so we ask d.gtld-servers.net where we learn that tom.com is delegated to four name servers, including edns.wyith.net (3), so we ask that one and get the following answer:
Tom.com is delegated to four domain name servers, however one delegated name server is missing in the zone and two other name servers are listed. The primary name server is ns1.tom.com. Incoming mail for tom.com is handled by one mail server at 163.net. tom.com has one IP number (22.214.171.124).