When connecting to an Exchange server using Outlook 2013 you may encounter an SSL trust error.
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.
If you choose Yes to proceed you may also encounter this additional error message.
There is a problem with the proxy server's security certificate. The security certificate is not from a trusted certifying authority.
Outlook is unable to connect to the proxy server [servername] (Error Code 8).
This error occurs when the Exchange server is configured with a self-signed SSL certificate.
Outlook makes connections to the Exchange server over HTTPS and therefore must trust the SSL certificate that is configured on the server, otherwise it will display those error messages to the end user.
To resolve the issue install a valid SSL certificate on the Exchange server from a trusted certificate authority. See Exchange Server 2013 SSL certificates for more details on this as well as step by step instructions.