This message occurs, when you are using a secure connection to the mail server, but the security certificate used by the server has a different name on it, to the name that Outlook Express expects. Because the whole idea of secure connections is to encrypt the conversation, and prove the identity of the remote computer - if the remote computer says it isn't they one that you expected, then this fundamentally defeats the security of the connection, therefore this error cannot be disabled without disabling security.
You should check that the right server name is configured in OE, if that doesn't solve it then you should check up with your ISPs (or the mail provider's tech support - it's probably a simple misconfiguaration on your end, but I have know it to be something on the other end: maybe they have installed the wrong certificate on their server).
Finally if all that fails, then you should disable security for that connection.