“Reverse DNS lookup ”在百度、谷歌等搜索裏面輸入這個單詞能看到好多網站提供反向DNS查詢，我個人感覺下面這個網站好用一些，http://www.dnswatch.info/
在第一個查詢框，也就是DNS Lookup / IP lookup這個下面輸入你的ip地址，這裏以58.251.139.211作例子，點擊查詢，出來的是一個結果頁面，查看最後面的Monitor performance and availability of your DNS Server (e.g. ns2.cnc-gd.net)這句話：
DNSWatch > Reverse DNS Lookup for 58.251.139.211
Searching for 211.139.251.58.in-addr.arpa. PTR record at E.ROOT-SERVERS.NET. [192.203.230.10] ...took 1 ms
Searching for 211.139.251.58.in-addr.arpa. PTR record at f.in-addr-servers.arpa. [193.0.9.1] ...took 8 ms
Searching for ns2.lacnic.net. A record at E.ROOT-SERVERS.NET. [192.203.230.10] ...took 1 ms
Searching for ns2.lacnic.net. A record at g.gtld-servers.net. [192.42.93.30] ...took 154 ms
Searching for ns2.lacnic.net. A record at dns.anycast.lacnic.net. [200.3.13.10] ...took 194 ms
Searching for 211.139.251.58.in-addr.arpa. PTR record at ns2.lacnic.net. [200.3.13.11] ...took 202 ms
Searching for ns2.cnc-gd.net. A record at E.ROOT-SERVERS.NET. [192.203.230.10] ...took 1 ms
Searching for ns2.cnc-gd.net. A record at k.gtld-servers.net. [192.52.178.30] ...took 2 ms
Searching for ns2.cnc-gd.net. A record at ns1.cnc-gd.net. [221.4.9.48] ...took 399 ms
Searching for 211.139.251.58.in-addr.arpa. PTR record at ns2.cnc-gd.net. [221.4.66.66] ...took 376 ms
Domain Type TTL Answer
211.139.251.58.in-addr.arpa. PTR 86400 reverse.gdsz.cncnet.net.
Monitor performance and availability of your DNS Server (e.g. ns2.cnc-gd.net) - starting at $1/month
Total elapsed query time: 1,338 ms
Since these results are absolutely up-to-date they may differ from the results of your local nameserver.
It can take up to the specified "time to live" (TTL) for your nameserver to update its cache.
DNS queries have been sent from Frankfurt am Main, Germany
這句話說的應該是你的IP的DNS Server是e.g. ns2.cnc-gd.net，請聯繫這個域名，作58.251.139.211這個地址的反向域名解析。
然後你可以要求你的IP提供商，即ISP運營商，有時也是你的主機託管商，給你的這個IP加上反向域名解析記錄，一般將這個IP反向解析成您的郵件服務器對應的mx記錄。
追加兩個簡單的測試反向DNS方法：
（一）命令：nslookup -qa=ptr 58.251.139.211 這個可以簡單看出結果
結果如下：
服務器: resolver1.opendns.com
Address: 208.67.222.222
非權威應答:
211.139.251.58.in-addr.arpa name = reverse.gdsz.cncnet.net
（二）命令：dig -x 58.251.139.211 +trace 這個加上+trace可以看到詳細的信息
; <<>> DiG 9.3.0beta2 <<>> -x 58.251.139.211 +trace
;; global options: printcmd
. 387585 IN NS a.root-servers.net.
. 387585 IN NS b.root-servers.net.
. 387585 IN NS c.root-servers.net.
. 387585 IN NS d.root-servers.net.
. 387585 IN NS e.root-servers.net.
. 387585 IN NS f.root-servers.net.
. 387585 IN NS g.root-servers.net.
. 387585 IN NS h.root-servers.net.
. 387585 IN NS i.root-servers.net.
. 387585 IN NS j.root-servers.net.
. 387585 IN NS k.root-servers.net.
. 387585 IN NS l.root-servers.net.
. 387585 IN NS m.root-servers.net.
;; Received 228 bytes from 208.67.222.222#53(208.67.222.222) in 46 ms
in-addr.arpa. 172800 IN NS f.in-addr-servers.arpa.
in-addr.arpa. 172800 IN NS e.in-addr-servers.arpa.
in-addr.arpa. 172800 IN NS d.in-addr-servers.arpa.
in-addr.arpa. 172800 IN NS c.in-addr-servers.arpa.
in-addr.arpa. 172800 IN NS b.in-addr-servers.arpa.
in-addr.arpa. 172800 IN NS a.in-addr-servers.arpa.
;; Received 421 bytes from 198.41.0.4#53(a.root-servers.net) in 265 ms
58.in-addr.arpa. 86400 IN NS ns2.lacnic.net.
58.in-addr.arpa. 86400 IN NS ns4.apnic.net.
58.in-addr.arpa. 86400 IN NS sec1.authdns.ripe.net.
58.in-addr.arpa. 86400 IN NS ns1.apnic.net.
58.in-addr.arpa. 86400 IN NS tinnie.arin.net.
58.in-addr.arpa. 86400 IN NS apnic1.dnsnode.net.
58.in-addr.arpa. 86400 IN NS ns3.apnic.net.
;; Received 220 bytes from 193.0.9.1#53(f.in-addr-servers.arpa) in 280 ms
251.58.in-addr.arpa. 86400 IN NS ns2.cnc-gd.net.
251.58.in-addr.arpa. 86400 IN NS ns1.cnc-gd.net.
;; Received 91 bytes from 200.3.13.11#53(ns2.lacnic.net) in 357 ms
211.139.251.58.in-addr.arpa. 86400 IN PTR reverse.gdsz.cncnet.net.
139.251.58.in-addr.arpa. 86400 IN NS dns2-ftcg.gdsz.cncnet.net.
139.251.58.in-addr.arpa. 86400 IN NS dns1-ftcg.gdsz.cncnet.net.
;; Received 130 bytes from 221.4.66.66#53(ns2.cnc-gd.net) in 46 ms