In this short guide, we will learn how to force Thunderbird to download all emails from your IMAP server for offline access, backup purposes, and faster searching. Whether you're switching email clients, creating local backups, or need offline email access during travel, downloading your complete email archive is essential for email management and data security.

By default, Thunderbird IMAP only downloads email headers, leaving message bodies on the server. This saves disk space but requires internet connectivity to read emails.

Problem: Incomplete Email Downloads in Thunderbird

Thunderbird's default IMAP behavior downloads only headers and recent messages, leaving older emails and attachments on the remote server. This causes:

  • No offline access to email content
  • Slow search requiring server queries
  • Incomplete backups when exporting profiles
  • Lost emails if server access is removed

Users need full local copies for archival, migration, or offline work.

Solution: Configure Thunderbird for Full Download

1. Enable Message Synchronization (All Folders)

Steps:

  1. Right-click on email account in folder pane
  2. Select Settings (or Properties)
  3. Click Synchronization & Storage
  4. Check "Keep messages for this account on this computer"
  5. Click Advanced button
  6. Select all folders you want to download
  7. Click OK → OK

Result: Thunderbird downloads complete message bodies for selected folders.

2. Change Message Retention Settings

Force download of all messages regardless of age.

Steps:

  1. Go to Settings → Account Settings
  2. Select Synchronization & Storage
  3. Under Message Synchronizing, click Advanced
  4. Check "Synchronize all messages locally"
  5. Uncheck "Don't download messages larger than"
  6. Click OK

Important: This downloads large attachments and may consume significant disk space.

3. Increase Default Download Limits

Remove age-based and size-based download restrictions.

Settings to modify:

  • Disk Space: Under Synchronization & Storage → Set to "No limit" or high value
  • Message Age: Ensure "Synchronize all messages" is selected (not "last X days")
  • Download Size: Uncheck "Don't download messages larger than X KB"

4. Force Immediate Synchronization

Manually trigger immediate download of all messages.

Steps:

  1. Right-click on folder (Inbox, Sent, etc.)
  2. Select Properties
  3. Check "When getting new messages for this account, always check this folder"
  4. Go to File → Offline → Download/Sync Now
  5. Select "Download selected folders"
  6. Click OK

Progress: Monitor download in status bar at bottom of Thunderbird window.

5. Check Storage Location

Verify downloaded emails are saved locally.

Profile location:

  • Windows: %APPDATA%\Thunderbird\Profiles\
  • Mac: ~/Library/Thunderbird/Profiles/
  • Linux: ~/.thunderbird/

Email storage: Each folder has corresponding .msf (index) and mbox file (messages).

6. Compact Folders After Download

Optimize local storage after downloading all messages.

Steps:

  1. Right-click on folder
  2. Select Compact
  3. Or go to File → Compact Folders

Benefit: Removes deleted message placeholders and reclaims disk space.