Preauth stands for pre-authentication, and is a mechanism to enable a trusted third party to vouch for a user’s identity. For example, if a user has already signed into a portal and wants to enter the mail application, they should not have to be prompted again for their password.
You can create preauth key by the following command.
zmprov gdpak domain.com
It is stored in a zimbra attribute. You can get it by the command,
zmprov getDomain testdomain.com zimbraPreAuthKey
If this returns null, then preauth key is not generated earlier. Else, it will return the preauth key of the domain.
More details can be read from http://wiki.zimbra.com/wiki/Preauth