What is Zimbra pre-authentication and how does it work?

Zimbra pre-authentication allows you to log into a user's mailbox using a pre-shared key (PreAuthKey) instead of a password. It works by computing an HMAC-SHA1 hash from the username, timestamp, and an expiration value, then passing this token in the URL or SOAP request. For a detailed walkthrough, see the Zimbra SOAP API Development Guide 6 - Pre-authentication.