‘550 no rDNS entry for xxx.xxx.xxx.xxx’ 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 :

user@destinationdomain.com>: 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 1.2.3.4, the receiver’s mail server will check the rDNS of 1.2.3.4 to check if the IP resolves to a valid hostname.

Let’s say John@abc.com sends an email to Anne@xyz.com. Here’s how the mail transfer happens:

John’s server (with IP 1.2.3.4) contacts Anne’s server.

John’s server: “Hey! I am from abc.com. I have a mail for Anne@xyz.com”.

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”mail.abc.com”), 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. myweb.com) points to your IP (eg. 1.2.3.4), and 1.2.3.4 points back to myweb.com.

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.

Conclusion

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.