Need help?

Our experts have had an average response time of 13.52 minutes in October 2021 to fix urgent issues.

We will keep your servers stable, secure, and fast at all times for one fixed price.

‘550 No rDNS entry for x.y.z.n’ explained and resolved

by | Sep 4, 2018

‘550 no rDNS entry for’ means that there is no reverse DNS record for the sending email server.

You may see this error in logs :

Tue 2018-09-04 10:36:46: <-- 550-Sending server has no reverse DNS (PTR record)

Or you may get a bounce back message :

[email protected]>: host mail.[destination_ip] said: 550 No rDNS entry for 100.200.300.400 (in reply to RCPT TO command)

It’s usually among VPS owners, Dedicated Hosts etc., that we have seen this error.

Let’s take a look at the ‘550 No rDNS entry for‘ error. First off, let’s understand the working of rDNS.


How rDNS works

With DNS, you map the Domain name to IP address. But with rDNS (reverse DNS), you map the IP address to the Domain name.

When you send a mail using a mail server with IP, the receiver’s mail server will check the rDNS of to check if the IP resolves to a valid hostname.

Let’s say [email protected] sends an email to [email protected]. Here’s how the mail transfer happens:

John’s server (with IP contacts Anne’s server.

John’s server: “Hey! I am from I have a mail for [email protected]”.

Anne’s server: “Hey! I have Anne’s email address here but let me check if you really are legit. Since I have your IP now, I’ll just do a rDNS on it”.

So, Anne’s server asks its DNS server, “Does this IP resolve to any domain”?

If DNS returns with a domain name (say””), then Anne’s servers tells John’s server,“Well, Your mail server is legit. I will deliver the email to Anne”.

If the DNS server returns no value (no results), then Anne’s server says :“Um..I am not sure I can trust you. So, I won’t be delivering the mail. Here’s the ‘550 No rDNS entry’ error”.


Why is rDNS used?

Almost all spams originate from infected PCs and consumer devices – there are millions of them.

Whereas legitimate mail servers are carefully administered, and will have an rDNS.

So, like in the above example, if the sender server doesn’t have any reverse DNS record at all, then it’s usually seen as a sign of spam and the email gets rejected.

Many mail server owners go a step further and sets up FCrDNS. Forward Confirmed Reverse DNS is a configuration where your domain (eg. points to your IP (eg., and points back to

This provides an additional level of legitimacy for the mail server, and improves the reputation score in spam filters.


How come there is no rDNS entry for certain servers?

Although effective, rDNS lookup can sometimes block valid email servers.

We’ve seen this happen in a few cases such as:

  • The sending server was assigned a new block of IPs fresh from ICANN, and rDNS is not set
  • The server was recently migrated to a new Data Center which doesn’t have rDNS set
  • Network delay or DNS error, causing lookups to fail

Here we’ll take a look at the two most common reason for this error.

But before that, let’s take a quick look at how rDNS is configured.

The rDNS record attached to an IP address is a PTR (Pointer) record. So, the company that owns this IP sets this record.

In most cases, it’s usually the hosting facility’s or ISP’s responsibility. So, any change to the reverse DNS PTR records have to be done by them.


1. No rDNS for the new IP addresses

Sometimes the network provider may give a set of new IP address block to a user.

Ideally, these IP addresses should have their PTR records configured in the DNS server of the hosting facility or ISP.

If it is not configured then the user’s email server will be seen as an illegitimate user and blocked.


2. After switching, new ISP (or Data Center) is yet to update the rDNS

It not uncommon to switch network providers.

But what we have seen is that, sometimes when the switch is made, the rDNS entries are not updated accordingly.

This again creates trouble as the new IP will not match to the hostname. So, even though the sending server is a valid user, it will still be blocked because the PTR update wasn’t made.


In both these cases, the user can’t do much other than contacting their network provider as the issue has to be fixed from their side.

At Bobcares, we avoid such situations by performing regular audits for the Web Hosts that we support. Whenever an update happens, we work with the IP providers to ensure that the PTR records are updated and properly maintained.



Today we have discussed about the uses of rDNS and the ‘550 no rDNS entry‘ that occurs with the missing rDNS entry. To resolve this issue, the corresponding network provider has to be contacted to update the PTR record in their DNS server.



Never again lose customers to poor server speed! Let us help you.

Our server experts will monitor & maintain your server 24/7 so that it remains lightning fast and secure./p>


var google_conversion_label = "owonCMyG5nEQ0aD71QM";


Submit a Comment

Your email address will not be published. Required fields are marked *


Privacy Preference Center


Necessary cookies help make a website usable by enabling basic functions like page navigation and access to secure areas of the website. The website cannot function properly without these cookies.

PHPSESSID - Preserves user session state across page requests.

gdpr[consent_types] - Used to store user consents.

gdpr[allowed_cookies] - Used to store user allowed cookies.

PHPSESSID, gdpr[consent_types], gdpr[allowed_cookies]


Statistic cookies help website owners to understand how visitors interact with websites by collecting and reporting information anonymously.

_ga - Preserves user session state across page requests.

_gat - Used by Google Analytics to throttle request rate

_gid - Registers a unique ID that is used to generate statistical data on how you use the website.

smartlookCookie - Used to collect user device and location information of the site visitors to improve the websites User Experience.

_ga, _gat, _gid
_ga, _gat, _gid


Marketing cookies are used to track visitors across websites. The intention is to display ads that are relevant and engaging for the individual user and thereby more valuable for publishers and third party advertisers.

IDE - Used by Google DoubleClick to register and report the website user's actions after viewing or clicking one of the advertiser's ads with the purpose of measuring the efficacy of an ad and to present targeted ads to the user.

test_cookie - Used to check if the user's browser supports cookies.

1P_JAR - Google cookie. These cookies are used to collect website statistics and track conversion rates.

NID - Registers a unique ID that identifies a returning user's device. The ID is used for serving ads that are most relevant to the user.

DV - Google ad personalisation

IDE, test_cookie, 1P_JAR, NID, DV, NID
IDE, test_cookie


These are essential site cookies, used by the google reCAPTCHA. These cookies use an unique identifier to verify if a visitor is human or a bot.