Explanation of reverse resolution

Due to the spread of spam, various anti-spam technologies have been proposed. Domain Name reverse resolution (rdns) is the most common type. More email servers need to check IP reverse resolution (PTR) records, especially foreign email servers, in many cases, the recipient rejects relay or cannot send the bounce message because there is no reverse resolution.
However, for various reasons, for example, reverse resolution charges are too high. ISP does not provide reverse resolution at all, and even ISP does not know what reverse resolution is .. Many Email System Administrators cannot record the PRT. What should I do?
Let me first introduce the exchange environment of our company. We now have a total of five domains, of which helo only uses one domain name, that is, I do not need to perform reverse resolution for my five domains and four domains. I only use one domain. In short, I only set the helo domain of the mail system as the PRT record you need.
For example, if your IP address is 211.12.123.2, you can try to set the helo domain to the following format:
2.123.12.211.in-ADDR. ARPA
This method is called reverse IP resolution spoofing. Some spammers use this method and are said to be able to bypass most reverse IP resolution checks. Of course, this is not an all-powerful method, but it is only a last resort, because according to the rfc821 standard and rfc1035 standard, such a helo domain is not regular (normal helo domain has correct domain name a records). If a policy like Sina needs to comply with both rfc821 and rfc1035 standards, check IP reverse resolution and helo information.
As far as I know, many people do not know about anti-resolution. I will popularize it.
1. What is reverse domain name resolution?
As we all know, at present, our company's Microsoft DNS does not have forward and reverse resolution, reverse resolution is to find the domain name through the IP address
2. How to perform reverse resolution
Contact your IP provider, such as China Unicom, China Telecom, and China Mobile, and call to ask what materials they need
3. Misunderstanding of sending Analysis
Reverse resolution is not used to prevent spam, but to prevent your emails from being used as spam.
4. Verify that reverse resolution takes effect
C: \> NSLookup
Default Server: sldc02.a.com
Address: 192.168.209.11


> Server 202.202.154.3
Default Server: [202.202.154.3]
Address: 202.202.154.3
> Set Qt = PTR # Remember that it must be in upper case. The lowercase prompt does not exist.
> Your server IP Address
The detailed information is as follows:
Non-authoritative response:
52.70.167.119.in-ADDR. Arpa name = mail.a.com
52.70.167.119.in-ADDR. Arpa name = mail. B .com

52.70.167.119.in-ADDR. Arpa name = mail.c.com

167.119.in-ADDR. Arpa nameserver = ns.sdjnptt.net.cn
167.119.in-ADDR. Arpa nameserver = dns-jn.sd.cninfo.net
Ns.sdjnptt.net.cn Internet address = 202.102.128.69
Dns-jn.sd.cninfo.net Internet address = 202.102.128.69

The following describes how NSLookup works.
6. Do I need reverse resolution for the most all fields in multiple domains?
Only reverse resolution of the helo domain is required.

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