This can be caused by using an incorrect host name, so first double-check the host name in the advanced identity/account settings (tap *Manual setup and account options*).
Please see the documentation of the email provider about the right host name.
Sometimes the right host name is in the error message.
<p>This can be caused by using an incorrect host name, so first double-check the host name in the advanced identity/account settings (tap <em>Manual setup and account options</em>). Please see the documentation of the email provider about the right host name. Sometimes the right host name is in the error message.</p>
<p><del>Another possible cause is <ahref="https://github.com/appmattus/certificatetransparency">Certificate transparency</a> failing, so try disabling it in the connection settings tab page.</del></p>
<p>You should try to fix this by contacting your provider or by getting a valid security certificate because invalid security certificates are insecure and allow <ahref="https://en.wikipedia.org/wiki/Man-in-the-middle_attack">man-in-the-middle attacks</a>. If money is an obstacle, you can get free security certificates from <ahref="https://letsencrypt.org">Let’s Encrypt</a>.</p>