Organizations migrating from Microsoft Outlook to Zimbra often face one technical challenge: how to move mailbox data stored in OST files into Zimbra. Since Outlook uses OST (Offline Storage Table) files and Zimbra supports the TGZ backup format, you can't import them directly.

OST files are copies of Exchange, Microsoft 365, or IMAP accounts that are synced on your computer. They depend on the profile and can't be uploaded to Zimbra directly. The mailbox needs to be exported to TGZ format so that Zimbra can read and restore the data.

This guide explains the complete process in a structured way.

Why Convert OST to TGZ for Zimbra?

Before getting into the steps, it's important to know why you need the TGZ format.

TGZ (Tar GZipped archive) files are used by Zimbra to:

  • Import backups of mailboxes
  • Bring back emails and folders
  • Keep the structure of the mailbox
  • Move information from one account to another

Because OST files can't be moved and Zimbra doesn't support them, exporting them to TGZ makes sure they work with Zimbra and that the mailbox is restored correctly.

Method 1: Manual Approach (Limited Conditions)

There is no way to export OST files directly to TGZ. You can try an indirect migration method using IMAP synchronization if you can get to Outlook, though.

Step 1: Set up your Outlook account

Check that the OST file is connected and that Outlook is working correctly.

Step 2: Add your Zimbra account to Outlook

Use IMAP settings to set up your Zimbra mailbox in Outlook.

Step 3: Drag and Drop Emails

Move mailbox folders or emails you choose from the OST mailbox to the Zimbra IMAP mailbox by hand.

Step 4: Allow Synchronization

Give Outlook time to sync data with the Zimbra server.

Limitations of the Manual IMAP Method

  • Needs an active Outlook profile.
  • Not helpful for OST files that don't have a parent.
  • Slow for mailboxes that are oversized.
  • Risk of incomplete synchronization.
  • Does not make a backup file in TGZ format.

This method only works when you can fully access Outlook.

Method 2: Step-by-Step Export of OST to Zimbra TGZ Format

When direct synchronization isn't possible, exporting the OST file to TGZ format is a reliable and structured way to solve the problem.

The ToolsGround OST to TGZ Converter lets users save mailbox data in a format that Zimbra can read.

Step-by-Step Guide

Step 1: Install the software and start it

Download and install the converter on your system.

Step 2: Add the OST file

Find the OST file you need on your computer and click on it.

Step 3: Scan the Mailbox Data

The software looks through the OST file and shows mailbox folders for you to look at.

Step 4: Select Mailbox Items

You can choose entire mailbox folders or just certain pieces of data based on your migration needs.

Step 5: Choose TGZ as Output Format

Choose the TGZ format from the list of export options.

Step 6: Set Destination Path

Pick the place where the TGZ file that has been changed will be saved.

Step 7: Start Export Process

Start the process of exporting and wait for it to finish.

When the process is done, you will have a TGZ file that Zimbra can use.

How to Import a TGZ File into Zimbra

After you get the TGZ file:

  • Sign in to your Zimbra webmail account.
  • Go to Preferences.
  • Click on Export/Import.
  • Choose Import.
  • Put the TGZ file on the internet.
  • Begin the process of importing.

Zimbra will get back emails, folders, and other mailbox data from the TGZ file.

Frequently Asked Questions (FAQs)

1. Can Zimbra directly open an OST file?

No, Zimbra does not support the OST format. Before you can import the file, you need to change it to TGZ format.

2. What if my OST file is orphaned?

Outlook can't open an OST file that is no longer needed. In these situations, migration requires exporting to the TGZ format.

3. Will folder structure remain intact after exporting to TGZ?

Yes. When done right, the original folder and mailbox structures are kept.

4. Is IMAP synchronization better than TGZ import?

IMAP sync only works when Outlook is open, and it may take longer for large mailboxes. TGZ import lets you move your data in an organized way based on backups.

5. Can ToolsGround OST to TGZ Converter export selected folders only?

Yes. It lets users pick certain folders or all of their mailbox data to export to TGZ format.

Conclusion

To export Outlook OST mailbox data to Zimbra TGZ format, you need to make sure the formats are compatible and plan ahead. IMAP synchronization only works in certain situations, and it only works when Outlook is open.

If you have large OST files or want to do the migration offline, exporting the mailbox to TGZ format is a simple and reliable way to do it and bring the data back into Zimbra.