According to the RFC https://tools.ietf.org/rfc/rfc821.txt the 400
series codes are for temporary issues ("4yz Transient Negative
Completion reply") so LISTSERV is doing exactly what it is supposed to
do - save the message and attempt delivery again later.

Perhaps the real question is: Why does your Postfix consider "domain not
found" to be a temporary problem? If it gave a 550 response then
LISTSERV would know not to retry the message.

I can't help further with that, I never used Postfix.

On 6/18/2019 10:18 AM, Jeff Beaird wrote:
> [snip]
>> It looks like Postfix is not sending a proper bounce message back to
>> LISTSERV for this problem.  As suggested by someone else, you should
>> communicate with the LISTSERV Admin about this.
> I have verified via tcpdump that the 450 error is being sent back to Listserv.  Is there a way I could see what Listserv is doing with that bounce message when it receives it?

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