Delivery has failed on the enclosed message for the following
reasons reported either by the mail delivery system on the mail
relay host or by the local TCP/IP transport module:

451 4.3.0 <ip184-182-116-20.lv.lv.cox.net[184.182.116.20]>: Temporary lookup failure

Sending with my primary ISP sending email comes back with this error? Works fine if I use my gmail account?

Dig command validates the ip name to the ipaddress?

Raw view of mesage has no headers??
From: Mail Delivery System <>
To: Self
Subject: Mail Delivery Failure.
Date: Wed, 22 Jan 2025 02:12:40 +1000
X-PMFLAGS: 128 0 1 9L3FJL4W.CNM

Delivery has failed on the enclosed message for the following
reasons reported either by the mail delivery system on the mail
relay host or by the local TCP/IP transport module:

451 4.3.0 <ip184-182-116-20.lv.lv.cox.net[184.182.116.20]>: Temporary lookup failure

dig returns ip info??
ip184-182-116-20.lv.lv.cox.net. 21600 IN A 184.182.116.20

I don't think this is the right place for answering your questions (if any, since I can't tell from your post as I don't understand what using a question mark is supposed to mean for you), Mike. Any hints from your provider?

Yesterday, was able to send messages with no issues, but now send message in queue and select send. Then get an new mesage that reports failure? Raw View shows.
From: Mail Delivery System <>
To: Self
Subject: Mail Delivery Failure.
Date: Wed, 22 Jan 2025 03:40:49 +1000
X-PMFLAGS: 128 0 1 RZ0DTNNH.CNM

Delivery has failed on the enclosed message for the following
reasons reported either by the mail delivery system on the mail
relay host or by the local TCP/IP transport module:

451 4.3.0 <ip184-182-116-20.lv.lv.cox.net[184.182.116.20]>: Temporary lookup failure

Your original mail message follows:

Not seeing any information on were the 451 error is coming from?
No headers in message even showing a connection attempt.
The ip address both name and number are linked to my cable modem, so not clear on what the temporary lookup failure would be coming from? Both dig and rdns check show matching info.

Sending mesage using my gmail account via same Pegasus has message go thru?

Did the logging, and found this in log.
09:24:01.535: << MAIL FROM:mikes@guam.net SIZE=976<cr><lf>
09:24:01.662: >> 250 2.1.0 Ok<cr><lf>
09:24:01.663: << RCPT TO:mikes@guam.net<cr><lf>
09:24:01.792: >> 451 4.3.0 <ip184-182-116-20.lv.lv.cox.net[184.182.116.20]>: Temporary lookup failure<cr><lf>
09:24:02.131: << QUIT<cr><lf>
09:24:02.280: >> 221 2.0.0 Bye<cr><lf>
09:24:02.281: --- Connection closed at 22 Jan 2025, 9:24:02.281. ---

So, it is making connection. Tried putting IP address in Host File, but it still shows same error.
Don't know if it is something from Cox.net or my ISP in Guam that is causing issue? Using my Gmail ID has no issue?

Didn't know the log would show additional info like this. Thanks.

Should have been Strange in title??
Sent messages to my ISP guam.net and Cox Tech Support, but never got an answer from either.
But next day tried again, and has 133 messaged downloaded and a number of messages sent with no issue?
So, not sure were the problem was, or what fixed it, but can now send messages with no issue.
Made a new log with sending test message, and no line about the lookup succeeding appears, so must assume that something is doing the check, and only on failure does it report.

Thanks for replies.

Not that it would help in finding the cause, but it was categorized as temporary and appears to have been exactly that. Possibly some kind of maintenance, and if looking at the meaning of lots of the typical error phrasings you will recognize that most of them don't really name an exact reason - which can be really frustrating not only for users but also for developers.

Thanks for the info.