Getting a lot of 600 Failed to connect to the recipient's mail server. No DNS information was found

Problem reported by Brian Bjerring-Jensen - VonBjerring GmbH - 9/11/2023 at 12:54 AM

Submitted

Remote Server returned: '600 Failed to connect to the recipient's mail server. No DNS information was found for the 'xx.xx' domain. Status: 544 5.4.4 Host not found (not in DNS).'

Getting a lot of these. Thought is was DNS related so I changed from our internal DNS to QUAD9 and OpenDNS.

Didnt help.

Any ideas?

9 Replies

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Kyle Kerst Replied

9/11/2023 at 6:24 AM

Employee Post

Hey Brian, sorry to hear you're seeing DNS errors. Can I have you submit a ticket on this so we can help take a look? We have a debug log or two we should be able to use to help track this down.

Kyle Kerst Lead Internal Network/System Administrator SmarterTools Inc. smartertools.com

Brian Bjerring-Jensen - VonBjerring GmbH Replied

9/11/2023 at 7:29 AM

Hi Kyle :)

Done.

Kyle Kerst Replied

9/11/2023 at 2:56 PM

Employee Post

Thanks Brian! I'll note the community post on the ticket (if you didn't already) so we can circle back here with an update when we get to the bottom of it!

Kyle Kerst Lead Internal Network/System Administrator SmarterTools Inc. smartertools.com

Duarte Replied

9/21/2023 at 6:26 AM

Hi,

We are using the latest build (8657) and we also just had an episode related to DNS queries.

We do not use the SmarterMail DNS cache. When we used nslookup on the server we were able to carry out queries, but in SmarterMail the spool accumulated messages, because were unable to query blacklists or discover the MX records of the domains for delivery.

We rebooted and the situation returned to normal...

Brian Bjerring-Jensen - VonBjerring GmbH Replied

9/21/2023 at 8:29 AM

Use the primary ip of the interface under SMTP out. :)

We havent had issues since.

Duarte Replied

9/22/2023 at 6:59 AM

Yes, we have this set on all servers. That's not it...

Furthermore, there are performance problems. We compared the average time of 3 servers in build 100.0.8251.30716 (Aug 4, 2022) with 3 servers in most recent build (100.0.8657.28945, 100.0.8657.28945 and 100.0.8601.21923) and the average time to consult blacklists increased significantly.

In the older build only 1 RBL has the average time longer than 500ms, in the most recent builds more than 18 RBLs are longer than 1000ms, including two with 7500 ms. (They are identical servers with the same anti-spam settings).

NetsDirekt Replied

11/8/2023 at 4:57 AM

I have the same problems. Is there a solution for this?

Duarte Replied

11/16/2023 at 3:08 AM

Hi,

There will only be a solution in the build which is still beta...

https://portal.smartertools.com/community/a95642/false-dkim-spf-and-reverse-dns.aspx
https://portal.smartertools.com/community/a95594/smartermail-8629-spf-fail.aspx

Kyle Kerst Replied

11/16/2023 at 10:47 AM

Employee Post

Just leaving a note here in case it helps! The 600 error originally noted above simply indicates a connectivity or lookup issue happened, and so isn't necessarily indicative that you're facing a known issue. I recommend checking the Delivery/SMTP logs for more information on the session that failed in order to determine the culprit. Setting your primary/secondary DNS servers in SmarterMail to these can help as well:

1.1.1.1 (Cloudflare DNS)

8.8.8.8 (Google DNS)

If you need help investigating please don't hesitate to open a ticket with us. Have a good one!

Kyle Kerst Lead Internal Network/System Administrator SmarterTools Inc. smartertools.com

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