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Friday, April 12th, 2019 3:46 PM

Email NDR because of supposed invalid PTR (rDNS) record.

Hello, I am a sysadmin trying to find out why our emails are being rejected by AT&T.


The email is being sent from $user@$ on a hosted Microsoft Exchange server mail.$ to the recipient $ and we are getting a 550 5.7.1 NDR. 

Sanitized version of the NDR:

This message was created automatically by the SMTP relay on mail.$
A message that you sent could not be delivered to all of its recipients.
The following address(es) failed:

 SMTP error from remote mail server after MAIL FROM:<$user@$>:
 host []:
 550 5.7.1 Connections not accepted from servers without a valid sender domain.alph754 Fix reverse DNS for $static.ip

I have verified:

  • The domain\ip is not on a public blacklist
  • The PTR (rDNS) record is correct
  • The original message was not a mass email, it was to one bellsouth email address

I have dealt with a similar issue for other ISPs. They required dual PTR records that consist of a standard ip → domain and ip → . The contents of the secondary record are unique to the ISP.

Comcast Example:

ip1= 12.345.678.9

ptr1= mail.$


ip2= 12.345.678.9



Sanitized nslookup:

PS C:\Users\LTJ_DNS> nslookup
Default Server:

> set type=ptr
> 12.345.678.9

Non-authoritative answer: name = name = mail.$


We have been able to email AT&T customers without isse for at least 5 years, but suddenly our messages are being rejected. We are not AT&T customers, but this seems to be a problem that is on AT&T's end. 

If we need additional ptr records, what are the requirements? What is the appropriate communication channel I need to seek?



Community Support


210.6K Messages

4 years ago

Hello @LTJ_DNS,

We understand that this situation can be frustrating, and we're here to help you back on track! The best place to receive the assistance you are looking for would be by contacting our AT&T Postmaster. We are confident that they will be able to sort out the errors that you have been receiving!

If you have any other questions about AT&T service, please let us know!

Alexander, AT&T Community Specialist

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