Migrate Zimbra to Office 365: Complete Guide for Bulk Mailboxes (2026)

Why Teams Move Zimbra Mailboxes to Microsoft 365

The triggers are usually practical:

• Retiring a self-hosted Zimbra server
• Consolidating mail after a merger or department change
• Bringing archived TGZ exports back into live mailboxes
• Standardizing users on Outlook and Exchange Online

The goal is to move the required mailbox data into Microsoft 365 while retaining the supported folder structure.

What Zimbra Mailbox Data Looks Like

Zimbra mailboxes contain email, contacts, calendars, and task items. When using IMAP, you will only have access to the email messages and folders of your mailbox. A TGZ file is an export archive that can contain various data items from a Zimbra mailbox.

Method 1: Manual Route Using Microsoft 365 IMAP Migration

This is the free baseline and the first option to check when you migrate Zimbra emails to Office 365 without extra software. It suits a mail-only move where you have the required credentials to access the source mailboxes.

Requirements:

• Microsoft 365 mailboxes created and licensed for every user
• Zimbra IMAP server address, port, and security settings
• Each mailbox’s email address, username, and password
• Exchange admin center access

Steps of the Manual Route:

1. Create a CSV file containing EmailAddress, UserName, and Password.
2. Use the Exchange admin center to select Migration > Add migration batch.
3. Choose Migrate to Exchange Online, then IMAP migration.
4. Create the IMAP endpoint using the Zimbra server details.
5. Upload the CSV, set any folders to exclude, and start the batch.
6. Monitor the batch, update your MX records, allow at least 72hours to sync, check that your emails have been migrated ok and delete your batch.

Handling TGZ Files Manually

The Microsoft IMAP batch will read directly from a live server, so you need to restore your TGZ file into a Zimbra mailbox to be able to see them. You will need access to a Zimbra environment to do this.

Limitations of the Manual Method

The manual route supports bulk mailbox migration, but managing credentials, batches, and folder exclusions can require additional administrative effort at scale. Microsoft’s IMAP migration documentation states the boundaries:

• Mail only: contacts, calendar items, and tasks don’t migrate.
• Item limits: Mailboxes have a maximum item limit, and no message can exceed 35 MB.
• Credentials: User or supported administrator credentials must be provided in the CSV, which requires careful preparation across multiple accounts.
• Folder control: You exclude folders when the batch is created, rather than picking them from a mailbox view.

Method 2: Import Zimbra to Office 365 With a Professional Tool

The DRS Softech IMAP Backup Tool is a practical solution that allows you to easily transfer mailbox data from your IMAP account to Office 365. It supports single or batch mailbox migrations using CSV files. Easily browse through your folders, choose the necessary emails with date and advanced filters, remove duplicate items, and skip previously migrated data. Save your attachments separately, retain your folder structure, and generate reports for your migration.

Feature of The Tool

Feature	What It Does
IMAP Login	Connects with server address, email, password or app password, and SSL/TLS port; Batch/CSV mode supports multiple accounts.
Folder Selection	Lists the folder tree with mail counts to select the required folders.
Microsoft 365 Sign-In	Single Login with OAuth 2.0, or Admin Login with Client Secret or Modern Auth
Tenant Mailbox List	Shows Total, Licensed, unlicensed, and shared counts, with search, filters, and Import CSV
1:1 Mapping	Maps each source account to a destination mailbox, with View Mappings and Clear All
Mail Filters	Skip Previously Migrated Data, Remove Duplicate Mail, Date Range and Advanced Email Filter
Migration Options	After Migration Cleanup, Save Attachments Separately, Custom Folder Name, Preserve Folder Hierarchy

The tool provides the required options to migrate Zimbra emails to Office 365 with folder selection, email filters, and duplicate removal.

Free Trial for a Quick Test

The trial version allows you to migrate up to 100 items per folder. Choose Continue Anyway on the Trial Restriction page to continue using your mailbox as a trial. Or Upgrade Now to use the licensed version.

Steps to Migrate Zimbra to Office 365 Using the Tool

1. Download and open the DRS Softech IMAP Backup Tool. Go to Source > IMAP Login and enter your Zimbra IMAP credentials and server details.
2. Verify the connection, fetch the email count, and select the required folders.
3. Choose Office 365 as the destination and sign in using Single Login or Admin Login for bulk migration.
4. Choose your destination mailboxes for 1:1 mailbox mapping.
5. Configure the filters and migration settings. Go to the trial or licensed version to begin your migration.
6. View the migration progress and ensure your emails and folders have been migrated to Office 365.

Mail only is supported by IMAP. Export your contacts as CSV and calendars as ICS from Zimbra and import into Outlook. 

If You Only Have Zimbra TGZ Files

The DRS Softech Zimbra Converter turns TGZ files into PST, MBOX, EML, MSG, PDF, and other formats, and it accepts several TGZ files in one run. Teams that export Zimbra to Office 365 from archives use it as the first of two stages.

1. Download and launch the converter.
2. Click on Select File and browse to find your TGZ file. Click on Open.
3. Preview your content, select your desired items, then click on Next.
4. Select a destination, choose where to save your data, apply any filters, and click on Convert.
5. Begin converting your data. Once successfully exported, you can view your report.

After converting the TGZ file into PST format, import the resulting PST file into the corresponding Office 365 mailbox using Outlook or another supported import method. The converter’s trial converts 25 items per folder, and its software guide covers each screen. For large PST volumes, Microsoft’s Import service in Microsoft Purview is the alternative.

Manual Route vs Dedicated Tool

Mailbox count and data format usually settle the choice. If you plan to export Zimbra to Office 365 in bulk, check the rows on mailbox pairing and repeat runs.

Requirement	Manual Method	DRS Softech IMAP Backup Tool
Mailbox pairing	CSV rows with EmailAddress, UserName and Password	Click-to-map 1:1, reviewed in View Mappings
Folder choice	Exclude folders at batch creation	Select folders on the Mailbox Structure screen
Folder structure	Supports folder migration	Preserve Folder Hierarchy option
Content controls	Folder exclusions	Date range, advanced filter, duplicate removal
Repeat passes	Batch keeps syncing until you stop it	Skip Previously Migrated on re-runs
Attachment handling	Migrates email attachments with messages	Option to save attachments separately
Cost	No extra software	Trial available; license for full runs

Real-World Scenario

For a Zimbra to Microsoft 365 Migration, an IT team is required to decommission a Zimbra server and migrate the department’s mailboxes to Microsoft 365. They connect using the Admin Login functionality of the DRS Softech IMAP Backup Tool. Each Zimbra account is mapped to its corresponding Microsoft 365 mailbox, and a pilot migration is performed. Migration reports are generated and used to review the data. The emails are migrated to the destination mailboxes.

Explore more with DRS Softech! Discover our complete overview of email migration services and solutions for different email platforms and file formats. Find the right solution to make your next email migration simple and seamless.

Conclusion

While manual IMAP batch methods are useful when you just need access to mail and have access to mailbox details, more control is needed when using folders, filters, multiple passes, and TGZ archives. DRS Softech offers an IMAP Backup Tool for live Zimbra servers and a Zimbra Converter for TGZ files. This lets you choose the right option to migrate Zimbra to Office 365 depending on your data format. Try the free trial on your pilot mailbox and purchase the license to complete the migration.

Frequently Asked Questions