Most SSL server certificates will only secure a single domain name or sub-domain. For example, a certificate could secure www.mydomain.com or mail.mydomain.com but not both. The certificate will still work on a different domain name but the web browser will give an error anytime it sees that the address in the address bar doesn't match the domain name (called a common name) in the certificate. If you need to secure multiple sub-domains on a single domain name, you can buy a wildcard certificate. For a wildcard certificate, a common name of *.mydomain.com would secure www.mydomain.com, mail.mydomain.com, secure.mydomain.com, and so on. There are also special certificates such as Unified Communications (UC) certificates for Microsoft Exchange Server 2007 that can secure several different domain names in one certificate.