Rouncube stops receiving emails after the server is upgraded to Ubuntu 24.04 LTS

After upgrading the Ubuntu server from 20.04 LTS or 22.04 LTS to 24.04 LTS My Roundcube stoped receiving emails any more. When checking the exim4 logs, I see those emails sent to me was frozen by the server.

The main problem is that while installing the upgrade, the file exim4.conf.template was moved to exim4.conf.template.dpkg-old and the upgrade installed the fresh exim3.conf.template file for me so that the previous configuration was gone.

To fix this, I’ve moved the new file to a backup and bring back the old file as the current configuration file:

cp /etc/exim4/exim4.conf.template /etc/exim4/exim4.conf.template.backup
cp /etc/exim4/exim4.conf.template.dpkg-old /etc/exim4/exim4.conf.template

Then restart the exim4:

systemctl restart exim4