This is because you are connecting to a shared mail server so the SSL certificate will have a generic address associated with it. That address won't match the address you are using to connect to the mail server if you are using mail.[your domain] as the server address, and that is what causes the warning.
The connection will still be successful though and the SSL certificate will encrypt your connection. You can just ignore the warning and it won't cause any problems. If you keep getting the warning and you want to prevent it from appearing then you can change the server address in your connection settings to smtp.stackmail.com.
