Dovecot `ssl_ca_file`

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:

On the line number indicated in the maillog message, you should see a line similar to this:

Comment out that line, and then copy & paste it with ssl_ca_file replaced by ssl_ca such that the lines should read, thus:

Now, click the button.

Finally, click to apply your changes.

Job done!