THE PROBLEM
The mail server's source IP address which is trying to send mail to parabon.com does not have proper reverse DNS entries setup or the DNS server for that mail server was not functioning at the exact instant that the mail was trying to be delivered to parabon.com. In other words the IP address from which the mail is being sent could not be converted back to a host name. The owner of that mail server and their ISP (not Parabon) needs to correct this issue in order for mail to flow properly.
HOW TO GET THIS FIXED
The ISP who provides the reverse-DNS mappings for the mail server in question (not Parabon) needs to setup a reverse DNS mapping for the IP address of that mail server so that other hosts on the internet that try and convert that IP number back to a hostname will get a result. This is usually only a few minutes of work.
To find out who you need to contact for a given IP address you can
visit the American Registery of Internet Numbers (ARIN) "whois" site at
https://search.arin.net/rdap/
and enter the IP address that was rejected into the "Search"
field... The contact information for the party in charge of that IP
space (aka, "Netblock") should then be displayed for you.
You are encouraged to call or email them, and request they set up a valid reverse-dns (PTR) record for the IP address of your mail server.
DETAIL
In general, computers on the Internet all have an Internet Protocol (IP) address, somewhat like a unique phone number, assigned to them. Each Internet Service Provider is responsible both for allocating these addresses to devices on their networks as well as for maintaining the mappings of these devices names to their IP address numbers and these IP numbers back to computer names.
In effect, ISPs assign devices something like a phone number from a bank of numbers that are in the realm of that ISP. They then need to provide a "411" service mapping a given devices name to that number as well as a reverse-411 mapping of IP address numbers back to device names.
Using this "phone number" analogy, an ISP will assign a customer the phone number of 555-1212 and then must put in a listing for "John Doe" in the 411 database so that anyone asking for John Doe's phone number will be provided with the answer "555-1212" AND anyone asking "whose number is 555-1212" will be provided with the answer "John Doe".
All legitimate mail servers on the Internet will have been assigned an IP address by their ISP and that ISP should have set up both a forward DNS entry (an entry for mail.mycompany.com which points to the IP address for that mail server) as well as a reverse DNS entry (an entry which given the IP address assigned to that host can return the name mail.mycompany.com).
Many computers on the Internet which are being misused to send SPAM do not have these two entries setup. In almost all cases email from these hosts is SPAM.
Due to the tremendous amount of SPAM on the Internet, Parabon, along with many other ISPs (like AOL for example) have elected to only accept mail from mail servers which have both a forward and reverse DNS entry.
It doesn't particularly matter what these names and IP addresses are as long as a query to convert a name to an IP address or a query to lookup the name associated with a particular IP address return some result other than "no entry found". As almost all legitimate ISPs and mail servers have already done this, this restriction has little to no effect on most mail.
In the event that a legitimate mail sender is getting blocked because proper forward and reverse DNS entries for their mail server have not been setup they should contact their ISP (not Parabon) and inform them that they are unable to send email from their mail server because proper forward and reverse DNS entries have not been setup. Once this is fixed email will work.
IMPORTANT NOTES
Parabon has no control over the forward and reverse DNS entries for third parties outside of our network. While we provide this informational document to assist anyone having trouble sending email to our systems because of a lack of proper DNS setup, we have no acesss or control over DNS records not associated with us. With respect to reverse-DNS records (sometimes called "PTR" records) we only have control over those reverse entries in our network space. You will need to contact the ISP in charge of PTR records for any other space. For lookups of who controls the records for other IP spaces visit www.arin.net.
CREDIT
This page was reprinted and adapted with permission from the wonderful write-up at https://isp.vail.net/reverse-dns.html
