Your email client should show you if you have encountered that identity (identities of course can and should rotate keys, etc, and identities should be also signed by the keys that correspond to the certs signed by the CAs for the website/service). At first you don't know much about it, but it's a signal. If you continue to bank with HSBC and you get emails from them, you'll be able to spot a fake and the email client can say that it's a likely fake. No need to say anything else when you can't say otherwise.