Updated FAQ

FAQ.md

@@ -413,6 +413,9 @@ See also [this FAQ](#user-content-faq16).
 <br />
 *... Invalid security certificate (Can't verify identity of server) ...*
 
+This can be caused by using an incorrect host name, so first double check the host name in the advanced identity/account settings (setup step 1/2).
+Please see the documentation of the email provider about the right host name.
+
 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 [man-in-the-middle attacks](https://en.wikipedia.org/wiki/Man-in-the-middle_attack).
 If money is an obstacle, you can get free security certificates from [Let’s Encrypt](https://letsencrypt.org).