Drag and Drop migration
A common mistake is to try to drag and drop emails, calendar or contact items from a PST file into a Zimbra profile or to use the Outlook import tools (file > import) – DO NOT DO THIS, as any corrupt or missing items may not get migrated.
Problem items will probably still show up in Outlook, but they will not be on the Zimbra server and this often goes unnoticed for some time after which new emails will have arrived into the mailbox making it impossible to conduct a fresh migration and other devices or computers will sync from the server and will not have the complete set of mail.
Zimbra provide the PST Import Wizard which logs every transaction and it is therefore possible to identify any items which have not been successfully migrated.
Details on using the Zimbra PST Import Wizard