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? ############################ To unsubscribe from the LSTSRV-L list: write to: mailto:[log in to unmask] or click the following link: http://peach.ease.lsoft.com/scripts/wa-PEACH.exe?SUBED1=LSTSRV-L&A=1