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SMTP Error

Posted on 2011-03-17
Medium Priority
Last Modified: 2012-08-14
We are getting the following error when sending to some messages seem to go but others queue up and never seem to send. Does anyone know what could be causing this message.

SMTP greeting from host read: 421 4.7.1 : (DNS:NR)
Question by:twanderson
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LVL 76

Expert Comment

by:Alan Hardisty
ID: 35161951
If you follow the link - it advises you that you don't have Reverse DNS Setup:

The Reverse DNS lookup for your IP address is failing. This could be a transient issue. Confirm the IP that sends your mail. Then check the rDNS of that IP using our troubleshooting tools. If it passes, please wait 24 hours and re-try before opening a support request.
rDNS must be in the form of a fully-qualified domain name. rDNS containing are not acceptable, as these are merely placeholders for a valid PTR record. rDNS consisting of IP addresses are also not acceptable, as they do not correctly establish the relationship between domain and IP address.
rDNS that may be similar to dynamic IP space (containing pool, dhcp, dyn, etc.) may be treated as suspect, and should therefore should be changed to reflect a fully-qualified domain name with standard reverse DNS.

You need to call your ISP and ask them to configure Reverse DNS on your fixed IP Address as something like - as long as resolves back to your fixed IP Address.

LVL 84

Expert Comment

by:Dave Baldwin
ID: 35161997
If you click on that link, it tells you exactly what the problem is.  Basically you must have a domain name that they can lookup before they will accept email from you.

Author Comment

ID: 35162062
Thank you Alan,

We have checked our reverse dns it is setup correctly. We had a problem with the reverse DNS earlier in the week and it has been resolved. The only domain that we cannot send to is All other mail domains work correctly. We currently have 400+ emails queued for, but sometimes email to works. We sent in a support ticket to the AOL Postmaster site 2 days ago and have not heard anything, we tried calling but just get there regular support and they just send us back to create a new ticket. We are at a point were we don't know what else to do, this seems to be a problem with but we can't speak with anyone there to even troubleshoot the issue.
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LVL 76

Accepted Solution

Alan Hardisty earned 2000 total points
ID: 35162068
If you have filled in the ticket - you simply have to wait for a reply and then they should unblock you.

It may take a few days - but it will happen.
LVL 76

Expert Comment

by:Alan Hardisty
ID: 35162077
I submitted a request on the 20th October and got the removal email on the 8th November, so you just have to be patient unfortunately.

Author Comment

ID: 35162131
We ran there reverse dns tools on aol postmaster site and we get a success. When I try to telnet 25 from mailserver we get an error that says connection to host lost.

I understand that this may be a waiting game, but it is hard to wait and watch your queue build up and eventually fail. Plus I would hate to wait 3-4 days only to find out there was something else I could have done to resolve the issue.
LVL 76

Expert Comment

by:Alan Hardisty
ID: 35162156
Configure a new SMTP Connector scoped for the AOL.COM domain and send the mail to your ISP's mail server (Smarthost).

This can be done temporarily and then when you get the all clear email from AOL, you can delete the SMTP Connector and return to using DNS for the AOL domain.

Author Closing Comment

ID: 35164928
It appears they want to be friends with us again. Email is now going to Thanks for your help!

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