In your maillog, at /var/log/maillog, you may see a warning error similar to this:
This is easily fixed in Webmin by going to Webmin > Servers > Dovecot IMAP/POP3 Server and clicking the [Edit Config Files] button:
Comment out that line, and then copy & paste it with
ssl_ca_file replaced by
ssl_ca such that the lines should read, thus:
ssl_ca = /etc/webmin/letsencrypt-ca.pem
Now, click the button.
Finally, click to apply your changes.