bounce on group but only from 1 single gmail sender

[nicodeceulaer]

I manage a small google workspace with 6 members.
It is configured for my own domain (noted as <domain> in the rest of this message).
There is a group 'info' created that forwards to all 6 members through the email address  info@<domain>
This has been working fine for multiple years.
Config for DKIM, SPF is all fine.
Groups are configured to allow the entire internet to post by email.
Now I have 1 outside user, that is using his  <BAD>@gmail.com email account to send to info@<domain>, and he gets a bounce message:

From: Mail Delivery Subsystem <mailer-daemon@googlemail.com>
Date: Sun, Feb 16, 2025 at 12:32 PM
Subject: Delivery Status Notification (Failure)

We're writing to let you know that the group you tried to contact (info) may not exist, or you may not have permission to post messages to the group. A few more details on why you weren't able to post:

 * You might have spelled or formatted the group name incorrectly.
 * The owner of the group may have removed this group.
 * You may need to join the group before receiving permission to post.
 * This group may not be open to posting.

If you have questions related to this or any other Google Group, visit the Help Center at https://support.google.com/a/enidonk.be/bin/topic.py?topic=25838.

Nobody else is experiencing this bounce behaviour when sending to info@<domain>
If I use any other account (<GOOD>@gmail.com), the message properly arrives in the info@<domain> and gets delivered to the 6 members.

I checked the Email log research, and found indeed that there is an event logged for the bounced email.
I've attached the email log for the bad gmail sender (which bounces) and the good gmail sender (which delivers).
I've went through the Q&A messages looking for things I can still investigate, but I am out of ideas at the moment. So looking here for some pointers on what else to do.

All help greatly appreciated,
Nico

bounce example

delivered example

[nicodeceulaer]

Some progress on my investigation. The bad sender gmail account is configured in Google Family as a minor account (under parental control). There is a chance this limits the possibilities to interface with google groups.
This linke seems to indicate this might be the problem:
https://support.google.com/mail/thread/26139840/child-gmail-account-has-no-access-to-mailing-lists-g...
The owner of that account is trying to fix this by raising the age of the account holder (it is a dummy account that has no physical person linked to it, the original creator thought it would be funny to put it under parental control of another account). Bumping a birthday of a minor account is protected by Google, so he changed the birthday to tomorrow. Hopefully starting tomorrow, we can remove the parental control and sending to the group email will work again.