“Internet Security Warning
The server you are connected to is using a security certificate that cannot be verified.
The target principal name is incorrect.”
Then make sure you have the SSL services for your account and its not expired. If you do not have any service then disable the SSL option in your email client.
Resolution : Please use None instead of Auto there in settings.
Please follow the steps.
Open email client.
Go to Tools >> Account Settings >> Select the account and Change ( or you can double click) . Now Click on More Settings >> Advanced and make sure BOX This server requires and encrypted connection ( SSL) is not checked and Use the following type of encrypted connection is set to None.