Plesk SSL certificate is loaded with domain through Nginx even if another one is configured in Plesk.
Steps to reproduce:
1. Enable nginx on server.
2. Configure signed proper certificate on domain.
3. Check domain certificate via openssl utility or some web checker(e.g. http://www.sslshopper.com/ssl-checker.html)
Plesk webmail certificate will be loaded.
Bug #PPPM-1561 . Fixed in Plesk 12 MU #65 and Plesk Onyx
Install the latest microupdates or upgrade to the latest Plesk version .