After installing an Exchange 2013 server into an existing organization Outlook clients may begin displaying a security alert warning dialog with the name of the Exchange 2013 server in it.
An example of the warning text is:
The security certificate was issued by a company you have not chosen to trust. View the certificate to determine whether you want to trust the certifying authority.
This can occur if the Outlook clients are connecting to Autodiscover or Exchange Web Services on the new Exchange 2013 server that you have installed.
This issue can be avoided if you review your Autodiscover configuration and make changes to the Autodiscover namespace configured on the server, as well as provisioning an SSL certificate for the new server.
See the following articles for more information: