The SSL certificate chain typically consists of:
ROOT Certificate
INTERMEDIATE Certificate
Domain / Webserver / Mailserver Certificate
The most common mistake is a missing "intermediate certificate". Please make sure you are using the correct intermediate certificate. The intermediate certificate / CA-bundle is usually delivered together with the SSL cert. Alternatively, you can visit out CA-Bundles website: https://www.interssl.com/en/knowledgebase.php?action=displaycat&catid=15