The mail server – source IP address which is trying to send mail to 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 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 VailNet) needs to correct this issue in order for mail to flow properly.


The ISP who provides the reverse-DNS mappings for the mail server in question (not VailNet) 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 and enter the IP address which was rejected in the “Search for:” dialog. The contact information for the party in charge of that IP space should then be displayed for you.

That party can then be called or emailed and a request to setup a valid reverse-dns (PTR) record for the IP address of the mail server can be made.


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-411mapping 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 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

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, VailNet/ColoradoNet 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 VailNet unless we are their ISP) 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.


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