If you are trying to configure Outlook to access Exchange Server and receive the following error when you enter your username and password, it means the security certificate used on the Exchange server is probably a certificate issued by the Exchange server organization instead of one purchased from a trusted certificated authority (such as Thwathe, Verisign, etc).
There is a problem with the server's security certificate. The security certificate is not from a trusted certifying authority. Outlook is unable to connect to the server www.domain.com. (Error Code 8).
Outlook will be unable to connect to the server until you trust the issuer by installing the certificate or the certificate is replaced with a certificate purchased from a trusted authority.
Trust the issuer
Trusting the issuer is as simple as adding the certificate to the Trusted Root Certification Authorities .
To trust the issuer, you need to be able to view the certificate and install it. If the dialog Outlook presents does not include a View Certificate or the certificate does not include an Install button, try logging into OWA from a web browser.
Step 1: View the certificate:
Step 2: Install the certificate:
Step 3: Browse to select the Trusted Root Certification Authorities store:
Moving a certificate
If the certificate is installed on your computer but is not in Trusted Root Certification Authorities, you can move it.
To do this, press Windows key + R to open the Run command, type certmgr.msc then press Enter.
Find the certificate and drag it to the Trusted Root Certification Authorities > Certificates folder.