Issue postfix/smtpd: Sender address rejected: Domain not found - how to whitelist?

[W4ru]

Server operating system version

Ubuntu 24.04.2 LTS

Plesk version and microupdate number

Plesk Obsidian 18.0.69 Update 3

Hello!
I am getting error messages for the receipt of a mail. In the past, messages have been delivered from exactly this recipient. I'm not sure (need to look in the logs again) if the mail server has been changed on the sender side.

postfix/smtpd [245148]

NOQUEUE: reject: RCPT from outbound-dkim.eu.khoros-mail.com[34.246.32.154]: 450 4.1.8 <[email protected]>: Sender address rejected: Domain not found; from=<[email protected]> to=<***> proto=ESMTP helo=<outbound-dkim.eu.khoros-mail.com>

Anyway: I have tried to set an exception using these instructions, as I was unsuccessful in greylisting the IP address.
All other mails are received, DNS config is ok and looks like this:

cat /etc/resolv.conf

nameserver 127.0.0.53
options edns0 trust-ad
search .

(https://support.plesk.com/hc/en-us/...deferred-emails-Host-or-domain-name-not-found)

This whitelist idea does not work:

Sender address rejected: Domain not found · modoboa modoboa · Discussion #2483

I have a daily report showing a specific e-mail address being rejected with the following message: 450 4.1.8 [email protected]: Sender address rejected: Domain not found; [email protected] to=loca...

github.com

How can I solve this problem? Thanks for any tips!