“Reverse DNS lookup ”在百度、谷歌等搜索里面输入这个单词能看到好多网站提供反向DNS查询，我个人感觉下面这个网站好用一些，http://www.dnswatch.info/
在第一个查询框，也就是DNS Lookup / IP lookup这个下面输入你的ip地址，这里以58.251.139.211作例子，点击查询，出来的是一个结果页面，查看最后面的Monitor performance and availability of your DNS Server (e.g. ns2.cnc-gd.net)这句话：
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
这句话说的应该是你的IP的DNS Server是e.g. ns2.cnc-gd.net，请联系这个域名，作58.251.139.211这个地址的反向域名解析。
然后你可以要求你的IP提供商，即ISP运营商，有时也是你的主机托管商，给你的这个IP加上反向域名解析记录，一般将这个IP反向解析成您的邮件服务器对应的mx记录。
追加两个简单的测试反向DNS方法：
（一）命令：nslookup -qa=ptr 58.251.139.211 这个可以简单看出结果
结果如下：
服务器: resolver1.opendns.com
Address: 208.67.222.222
非权威应答:
211.139.251.58.in-addr.arpa name = reverse.gdsz.cncnet.net
（二）命令：dig -x 58.251.139.211 +trace 这个加上+trace可以看到详细的信息
; <<>> 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
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