[MM3-users] uncaught bounce messages

I'm getting (many) uncaught bounce messages for one user. Other than forcibly unsubscribing them, is there anything I can do? I hesitate to post this message publically, but here is what I can see, somewhat anonymized

The MIME structure looks like

└┬╴multipart/mixed 7030 bytes ├─╴text/plain 442 bytes ├─╴message/delivery-status 176 bytes └─╴text/rfc822-headers 5301 bytes

In /var/log/mailman3/bounce.log I see lines like

Oct 23 08:20:00 2021 (5320) Bounce message w/no discernable addresses: <hex-digits@foo.bar>

In the delivery-status part I see

Final-Recipient: rfc822; fub@foo.bar Action: delayed Status: 4.0.0

The initial text/plain part looks like

Hi!

This is the MAILER-DAEMON, please DO NOT REPLY to this email.

A message is delayed for more than 4 hours for the following
list of recipients:

fub@foo.bar: Network error on destination MXs

Please note that this is only a temporary failure report.
The message is kept in the queue for up to 7 days.
You DO NOT NEED to re-send the message to these recipients.

Below is a copy of the original message: