Please check once more the log files, maybe there’s another letsencrypt/cert error?
With the update from traefik v2 to v3 just the uploaded and requested certs from the TLS settings page are shown. The app certs are not shown to avoid misconfigurations, see NethServer project milestone 8.4 - #10 by davidep
Did you install the latest updates in Software Center?
Let’s check if the mail domain is in the default certificate:
runagent -m traefik1 cat configs/_default_cert.yml
Is DNS working correctly for the used mail domain? Are the ports correctly forwarded?