How do you safely backup a large IMAP mailbox without corrupting emails or missing folders?

Steven Miller 60 Reputation points
2026-07-29T04:58:20.5733333+00:00

Our company is planning to migrate several IMAP email accounts to a different platform, but before making any changes we want to create a complete offline backup. The challenge is that some mailboxes are over 100 GB and contain thousands of folders, shared mailboxes, archived emails, and important attachments collected over many years.

We tested Outlook and Thunderbird for downloading emails locally, but the process often stops midway due to connection issues, and sometimes the folder hierarchy or read/unread status doesn't remain intact. Restarting the backup from scratch every time is also very time-consuming.

I'm looking for a reliable solution that can:

  • Resume interrupted backups instead of starting over
  • Preserve folder hierarchy, metadata, and attachments
  • Export emails into formats like PST, MBOX, or EML
  • Handle large IMAP accounts without performance issues
Microsoft 365 and Office | Subscription, account, billing | For business | Windows

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Rohit Raj Kumar 100 Reputation points
2026-07-30T06:41:52.6166667+00:00

Dear @Steven Miller

For very large IMAP mailboxes, I recommend creating a complete offline backup before making any migration changes. While email clients like Outlook and Thunderbird can synchronize IMAP accounts, they rely on continuous network connectivity and can take a significant amount of time to download mailboxes that exceed 100 GB. Microsoft also recommends downloading the complete mailbox before exporting it to a PST file to ensure all available data is included.

1. Prepare the backup environment

  • Verify that the destination drive has enough free storage for the entire mailbox.
  • Use a stable, high-speed internet connection to reduce the risk of synchronization failures.
  • Keep the computer powered on and disable sleep mode until the process finishes.

2. Configure the IMAP account

  • Add the IMAP account to your preferred email client.
  • Enable synchronization for all folders, including archive folders and custom folders.
  • Configure the client to download the entire mailbox instead of only recent messages.

3. Allow synchronization to complete

  • Wait until every folder finishes syncing before creating the backup.
  • Avoid closing the email client or interrupting the internet connection during the download process.
  • Large mailboxes may require several hours or even days, depending on mailbox size and network speed.

4. Export the mailbox

  • After synchronization completes, export the mailbox to the format that best fits your requirements:
    • PST for Outlook or Microsoft 365 environments.
    • MBOX for compatibility with many desktop email clients.
    • EML if you need individual email files for long-term storage or selective recovery.

5. Verify the backup

  • Compare the total folder count with the original mailbox.
  • Confirm that message counts match across several folders.
  • Open a sample of emails from different folders and verify attachments, timestamps, sender information, and read/unread status.
  • Store the backup on a separate drive or another secure location to protect against hardware failure.

Things to Keep in Mind

For mailboxes larger than 100 GB with thousands of folders, this process can become challenging. Long synchronization sessions may fail because of temporary network interruptions, server timeouts, or client limitations. If synchronization stops unexpectedly, some email clients may require additional time to determine what has already been downloaded, making the overall process longer. You should also validate the backup carefully before deleting or migrating the original mailbox to ensure no folders or messages were missed.

For additional guidance, Microsoft's documentation on exporting Outlook data and creating Outlook Data Files provides useful best practices:

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  1. Sophie N 18,175 Reputation points Microsoft External Staff Moderator
    2026-07-29T05:49:12.6+00:00

    Dear @Steven Miller,

    I understand the challenge you are facing. Standard email clients like Outlook and Thunderbird are designed for daily synchronization rather than bulk data archiving. When handling mailboxes over 100 GB with thousands of folders, desktop clients frequently hit connection timeout limits, run into memory overhead, or corrupt local cache files, forcing you to start over.

    To perform a reliable, resume-capable offline backup without missing folders or corrupting metadata, here are the most effective solutions categorized by approach:

    Solution 1: Create an Offline Backup Using IMAP Backup Tools (Recommended for Backup Requirements)

    If your target platform is Microsoft 365, you do not need to download local PST files first. Microsoft’s native IMAP Migration service in the Exchange Admin Center (EAC) connects directly from server to server.

    • Why it solves your problem: It uses delta-syncing (incremental migration). If a network connection drops, the service automatically resumes where it left off. It preserves folder structure, email flags, and attachments without requiring local mailbox downloads or PST creation.
    • How to implement: Create an IMAP migration batch in the Exchange Admin Center > Migration > Add migration batch > IMAP migration.
    • Official Microsoft Guidance: Refer to Microsoft’s official documentation on Migrating IMAP Mailboxes to Microsoft 365.

    Solution 2: Command-Line Engine (imapsync or imap-backup)

    If you require a local offline backup (in MBOX, EML, or server-level format), open-source command-line tools outperform desktop clients because they track state item-by-item rather than re-downloading folders.

    • imapsync (Server-to-Server or Local Mirror): The industry standard for large IMAP syncs.
      • Resuming: Uses unique message IDs (UIDs) to skip already downloaded messages seamlessly if interrupted.
      • Integrity: Preserves all folder hierarchies, flags (read/unread, flagged), dates, and attachments.
    • imap-backup / OfflineIMAP: Downloads IMAP mailboxes directly to local Maildir/MBOX structures with automatic reconnect and resume capabilities.

    Solution 3: Dedicated Enterprise Backup Utilities (For PST Splitting & EML/MBOX Export)

    If your requirement strictly demands PST, EML, or MBOX files:

    1. The PST Limit Warning: Outlook PST files have a default limit of 50 GB. Exporting a mailbox significantly larger than 50 GB into a single PST is not recommended because Outlook performance may degrade and the PST can approach supported size limits.
    2. Specialized Tools: Several third-party commercial tools, such as (names of commercial 3rd party apps removed by Moderator), offer IMAP backup and export capabilities. Customers should evaluate vendor documentation and test functionality before production use.
    3. Key Features to Look For:
      • Automatic PST Splitting: Automatically splits exports into 20 GB or 50 GB chunks.
      • Incremental Sync / Resume: Only pulls missing or new items during retry attempts.
      • Metadata Retention: Maintains folder paths, internet headers, and read/unread status.

    If you have any updates or further questions, please feel free to reply here, I’ll do my best to support you however I can. 

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