How to Migrate from Office 365 to Zimbra

Migrating from Office 365 to Zimbra involves moving mailbox data from Exchange Online to prepared Zimbra accounts. For email, EdbMails uses the Office 365 to IMAP workflow because Zimbra provides IMAP access to mailbox content. If contacts and calendars are also in scope, the Zimbra environment must provide the required CardDAV and CalDAV services and those endpoints must be configured in the migration job.

This page focuses specifically on the Office 365 → Zimbra direction. For broader Microsoft 365 migration capabilities, see the Office 365 migration Tool.

Why the Office 365 to IMAP Workflow Is Used for Zimbra

Zimbra exposes mailbox email through IMAP, so the EdbMails Office 365-to-Zimbra workflow uses an IMAP connection for email migration. This is why the application asks you to select Office 365 to IMAP Migration even though the actual destination is Zimbra.

IMAP is the email transport layer in this workflow. Contacts and calendars are different workloads: use the EdbMails IMAP with CalDAV/CardDAV Migration CSV File option only when the destination Zimbra environment supports the required CalDAV/CardDAV services and you have the corresponding connection details.

Office 365 to Zimbra Migration Methods

There is no standard Microsoft 365 export process that directly creates a complete Zimbra-native mailbox package. For email, IMAP is a practical interoperability path because Zimbra supports IMAP-based migration. Other approaches, including PST-based workflows, can be relevant in some scenarios but require separate export/import or conversion steps.

What Can Be Migrated Through Each Connection?

Office 365 to Zimbra Migration Prerequisites

Office 365 Source Readiness

• Inventory the Office 365 mailboxes and data volume included in the migration.
• Use the Microsoft 365 connection method supported by the current EdbMails workflow.
• If automatic Entra ID application registration is used, sign in with an account that has the administrative permissions required for that registration workflow.
• For manual application registration, use the permissions and account model required by the current EdbMails authentication configuration.
• Confirm mailbox access, MFA requirements, and any Conditional Access controls that could affect the migration.
• For large messages, compare the Exchange Online message-size limits with the Zimbra target limits; increasing a source-side limit does not guarantee that Zimbra will accept the same message size.

Zimbra Target Requirements

• Create the required Zimbra accounts before the migration.
• Confirm that IMAP is enabled and accessible for the target accounts.
• Verify the Zimbra IMAP hostname and the secure connection settings used by your environment.
• Confirm TLS/SSL requirements, certificate validity, authentication method, and firewall access.
• If contacts are in scope, confirm the CardDAV endpoint, credentials, and target address-book/folder details required by the EdbMails CSV workflow.
• If calendars are in scope, confirm the CalDAV endpoint, credentials, and target calendar/folder details required by the EdbMails CSV workflow.
• Check target mailbox storage and message-size limits.
• Test one Zimbra mailbox before starting a bulk migration.

Office 365 to Zimbra Folder Mapping

Office 365 and Zimbra can represent standard and special-use folders differently. Before a bulk migration, test how Inbox, Sent, Drafts, Trash, Junk, Archive, nested folders, and custom folders are mapped at the destination.

Review both mailbox mapping and folder behavior before starting the full job. Where the application suggests an automatic mailbox match, verify it; use manual mapping when a source mailbox must point to a different Zimbra account.

Recommended Staged Migration Approach

1. Pilot: Migrate a small representative set of users and validate Zimbra connectivity and folder behavior.
2. Initial migration: Transfer the existing mailbox data in scope.
3. Validation: Check email, attachments, folder structure and, when configured, contacts and calendars.
4. Incremental pass: Process newly added or changed items according to the current migration configuration.
5. Cutover: Change production mail routing to Zimbra after the target environment is ready.
6. Final validation: Verify new inbound/outbound mail and recheck migrated data after the routing change.

1. How to Migrate from Office 365 to Zimbra Using EdbMails

   • Step 1: Download and install EdbMails application

     • Download EdbMailsSetup.exe and follow the installation wizard.

        Refer to EdbMails system requirements for Office 365 migration.

     • After installation, click ‘Login’ or ‘Start Your Free Trial’ to access the application.

       

   • Step 2: Select Office 365 to IMAP Migration option

     • Choose ‘Office 365 Migration’ from the main dashboard.

       

     • Select ‘Office 365 to IMAP migration’ and enter a job name or keep the default one.
     • Click ‘Next’ to continue.

       

   • Step 3: Connect to source Office 365 server

     • Click ‘Add New Connection’ to establish a new connection or use ‘Connect to Existing’ for a saved connection.

       

     • Choose connection options, then click ‘Next’.
     • Select the login method and authenticate using Microsoft secure sign-in.

       

     • After authentication, load mailboxes manually or via a CSV file.
     • Select mailboxes and click ‘Next’.

       

   • Step 4: Connect to target Zimbra server

     • Select ‘Connect to IMAP’ for the Zimbra email migration.
     • For a single user, choose ‘Single User / Account Migration’, enter credentials, and click ‘Next’.

       

     • For bulk migration, choose ‘Multiple Users / Accounts Migration’.

       

     • Then click ‘IMAP Migration CSV File’ to download the sample CSV.

       

     • Open the downloaded CSV file and fill in the required details, including the email address (mailbox name), password, host name, and port. You can mark unused fields as ‘NA’. After updating the information, save and close the file.

       

     • Click on ‘Load Modified CSV File’ to import your updated CSV file. Verify the loaded mailboxes to ensure accuracy, then proceed by clicking ‘Next’ to continue.
     • To transfer emails, calendars, and contacts from Office 365 to the Zimbra Mail platform that supports CalDAV and CardDAV, choose ‘IMAP with CalDAV/CardDAV Migration CSV File’ and continue with the process.

       

     • A prompt will appear to download a sample CSV file. Select a preferred location on your computer and save it for further use.
     • Open the saved CSV file and fill in the necessary details such as Source Mailbox Name, Password, Host, Port, CalDAV and CardDAV credentials including usernames, passwords, servers, and folders. Once all fields are updated, save the file and close it.

       

     • Click ‘Load Modified CSV File’, navigate to the location where you saved the updated CSV, and import it into the application to move forward with the migration.

   • Step 5: Office 365 to Zimbra mailbox mapping

     • Use the EdbMails mapping screen to review the Office 365 source mailboxes and their corresponding Zimbra accounts. Where automatic mapping is available, verify the result before migration; use manual mapping when a source mailbox must be matched to a different target account.

       

   • Step 6: Start Office 365 to Zimbra migration operation

     • Click ‘Start Migration’ to initiate the transfer process.
     • After the initial migration, validate the migrated data and run any required incremental pass. Update MX records only when the Zimbra environment is ready to receive production mail.

       

Post-Migration Validation

• Compare representative messages, attachments, dates, and folder placement with the Office 365 source.
• Review the EdbMails migration report for completed, skipped, failed, or unmapped items.
• Verify mailbox mapping for every migrated user.
• If CardDAV is configured, validate representative contacts in Zimbra.
• If CalDAV is configured, validate representative calendar events in Zimbra.
• Test Zimbra client access and user sign-in.
• Keep the Office 365 source available until migration validation and cutover checks are complete.

Mail Routing and DNS Cutover

Do not change the MX record simply because the initial data copy has finished. Complete validation and any required incremental pass first. When the Zimbra environment is ready to receive production mail, update the domain's MX records and verify delivery.

• Confirm the target Zimbra mail domain and the MX values required by that environment.
• Review SPF for the new outbound-mail path.
• Review DKIM signing and publishing requirements.
• Review DMARC alignment after the mail-flow change.
• Verify outbound routing from Zimbra.
• Test inbound delivery after the MX change.
• If appropriate for the organization's DNS plan, reduce relevant TTL values ahead of the scheduled cutover.

When Is a Migration Tool Useful?

Office 365 to Zimbra Migration Limitations

• IMAP is an email-focused migration path and does not represent every Microsoft 365 workload.
• Contacts and calendars depend on the required CardDAV/CalDAV destination configuration when those workloads are included.
• Source and target message-size limits can differ.
• Folder names and special folders may require mapping validation.
• Mailbox permissions and collaboration settings require separate target-side configuration.
• Migration duration depends on mailbox size, item count, network conditions, Zimbra performance, Microsoft service limits, and selected concurrency.

Related Office 365 Migration Scenarios

Use the destination-specific page that matches the direction of the project:

• For the reverse direction, see migrate from google zimbra to office 365.
• For Microsoft 365 to supported Exchange versions, see office 365 migration to exchange 2016 2019.
• For the current Exchange migration workflow, see Office 365 to Exchange migration.
• For Zoho as the destination, see office 365 to zoho mail migration.

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Additional resources:

• Office 365 to Yahoo Mail migration
• Office 365 to cPanel Migration
• Office 365 to SmarterMail Migration
• Office 365 to IceWarp migration