That is unrelated and you're seeing it as a result of the server's certificate having been renewed. iOS 10.3 has no issues with the certificate. If you're using your own domain as the mail server, this will naturally not match the server's certificate and result in the trust issue. You can change your mail server to the hostname of your server instead to avoid the mismatch. The issue with more recent iOS versions is that it often doesn't give you the option to "Continue" as it did before. If you require further assistance please submit a ticket for support.