Migration to XMission Zimbra
Migration to XMission Zimbra is typically straight forward for most customers.
Migration and Import Tools in Admin Panel:
Zimbra provides a number of quality migration tools that can be found inside the Domain Administrator control panel. https://zimbraadmin.xmission.com
Microsoft Outlook PST Import Tool
This tool allows users to import Microsoft Outlook PST files to the Zimbra Server.
Migration Wizard for Microsoft Exchange This application performs a server-to-server migration of email, calendar and contacts from Microsoft Exchange to the Zimbra Server. https://wiki.xmission.com/Zimbra_Hosting_Admin#Importing_Accounts_from_Exchange
IMAPsync Tutorial by XMission
Highly accurate way to migrate mail data: https://wiki.xmission.com/Zimbra_Migration:_IMAPSync
Pay particular attention to the IMAPsync tips and tricks section of the article.
Non-mail Data Import to Zimbra
Zimbra.com wiki article on how to deal with non mail data: https://wiki.zimbra.com/wiki/Calendar_and_Contacts_Migration
Google Gsuite Migration Tutorial
Google Apps migration to Zimbra with XMission using Oauth and the IMAPsync https://xmission.com/blog/2015/09/09/migrate-your-email-from-google-apps-to-zimbra-collaboration