End of Life for ZCS v5.x
Zimbra v5, which was originally released in December 2007, will reach End of Life (EOL) status on March 31, 2012. This means that Zimbra technical support will no longer be available for any customers still running any version of ZCS 5. after that date. Customers are encouraged to upgrade to ZCS v7.2 to enjoy the benefits of the new features and support options.

More details can be found here, » Zimbra :: Blog
What is the point of suggesting people upgrade to a version of software that does not exist - and thus cannot be tested in advance of a production upgrade ?