Using Work Offline in Thunderbird or Betterbird can be used to force the client to synchronise and keep a local copy of mail that is already visible on the server. This is most useful when preparing to work without an internet connection, or when making sure messages are cached locally before travel or maintenance.

What Work Offline does

When Work Offline is enabled, Thunderbird/Betterbird stops communicating with the mail server and uses only locally stored data.

Used together with synchronisation settings, it can help ensure that:

• message headers and message bodies are downloaded to the computer
• folders are available without an internet connection
• previously synced mail can still be opened while offline

Important points:

• Work Offline by itself does not magically download every message unless the account is already configured to keep messages on this computer.
• For IMAP accounts, full download depends on synchronisation settings.
• For POP accounts, messages already downloaded are normally already stored locally.
• Very large mailboxes may take considerable time to fully sync.

Before using Work Offline

First make sure the account is configured to keep full copies of messages locally.

Thunderbird / Betterbird – enable synchronisation for an IMAP account

1. Open Account Settings.
2. Select the relevant email account.
3. Click Synchronization & Storage.
4. Ensure Keep messages in all folders for this account on this computer is ticked.

To make sure all folders are included:

1. In the same Synchronization & Storage section, click Advanced…
2. Tick all folders that should be available offline.
3. Save the changes.

To avoid Thunderbird only keeping recent mail, check retention settings:

• In Synchronization & Storage, review any options limiting storage by age or size.
• If there is an option such as keeping only recent messages, disable it if a full mailbox download is required.

Recommended method to force download of all mail

Method 1: Enable synchronisation, then use Work Offline

This is the usual method for downloading all currently selected mail folders.

1. Confirm synchronisation is enabled for the account.
2. Open each important folder at least once, especially:

• Inbox
• Sent
• custom folders
• archive folders
• subfolders

1. Wait while Thunderbird/Betterbird finishes background activity.
2. Use the menu:

• File
• Offline
• Work Offline

1. If prompted about downloading messages for offline use, choose the option to download or synchronise now.
2. Leave Thunderbird/Betterbird running until activity stops.

After the download finishes:

1. Stay in offline mode.
2. Open several folders.
3. Open a few older messages and messages with attachments.
4. Confirm that message bodies open without needing a server connection.

If messages open correctly while offline, they are stored locally.

Method 2: Download specific folders for offline use

If only some folders need to be fully available offline:

1. In the folder list, right-click the required folder.
2. Open Properties.
3. Enable the setting for offline use, usually labelled similarly to:

• Select this folder for offline use

1. Repeat for each required folder.
2. Then use:

• File
• Offline
• Download/Sync Now if available, or
• File
• Offline
• Work Offline

This is useful where only selected folders should be cached.

Method 3: Use Download/Sync Now before going offline

Some versions show a separate command which is often more direct than simply toggling Work Offline.

1. Open:

• File
• Offline
• Download/Sync Now

1. Tick the options to download messages for offline use.
2. Include the required accounts and folders.
3. Start the synchronisation.
4. Wait for completion.
5. Then switch to:

• File
• Offline
• Work Offline

Where available, this is often the clearest way to force a sync before disconnecting.

How to tell whether all mail has downloaded

A folder may show all message headers but still not have all message bodies stored locally. To check properly:

• Disconnect from the internet, or enable Work Offline
• Open old emails from several dates
• Open emails from different folders
• Open messages that were not viewed before
• Check whether attachments open

If a message body is blank or Thunderbird says it needs to connect, that message has not yet been fully cached.

If only headers are visible

This usually means the account is IMAP and full synchronisation is not yet enabled or not yet complete.

Check the following:

1. Synchronization & Storage is enabled for the account.
2. The folder is selected for offline use.
3. No age limit is restricting local copies.
4. Thunderbird/Betterbird has been left running long enough to finish syncing.
5. The folder has been clicked and allowed to load fully.

For large folders, it may help to:

• sort by date and scroll through older mail
• click into each major folder and subfolder
• leave the program open overnight

If the menu path is missing or different

Menu layout can vary slightly between Thunderbird versions and Betterbird builds.

Common ways to access the function:

• classic menu bar:
• File
• Offline
• Work Offline
• app menu:
• menu button
• look for File or Offline
• by right-clicking folders and enabling offline use in folder properties

If the menu bar is hidden, press:

Alt

This normally shows the classic menu bar temporarily on Windows.

Best practice for downloading an entire mailbox

For the most reliable result:

1. Enable account-wide synchronisation.
2. Include all folders in Advanced synchronisation settings.
3. Disable any setting that limits downloads by age.
4. Open Thunderbird/Betterbird and leave it online until background activity finishes.
5. Use Download/Sync Now if available.
6. Switch to Work Offline.
7. Test several old emails while offline.

Notes for POP accounts

For POP mail accounts:

• messages are usually already downloaded to the computer when received
• Work Offline mainly just prevents server access
• there is usually no separate need to force-download old mail unless the messages were never collected onto that computer

If old messages are not present in a POP account, they may not exist on that device unless they were downloaded previously.

Limitations and cautions

• This process only downloads mail that the account can currently see on the server.
• If the server does not show older mail, Thunderbird cannot download it.
• Shared mailboxes, very large folders, or slow connections may take a long time.
• Attachments are only available offline if the full message has been synced.
• Disk usage can increase significantly for large mailboxes.

Quick summary

To force Thunderbird or Betterbird to download all email for offline use:

1. Open Account Settings.
2. Enable Keep messages in all folders for this account on this computer.
3. In Advanced…, select all required folders.
4. Remove any age-based retention limit.
5. Use File > Offline > Download/Sync Now if available.
6. Then use File > Offline > Work Offline.
7. Test old messages while offline to confirm they are stored locally.

The key point is that Work Offline is the final offline state; the actual downloading depends on the account’s synchronisation settings being enabled first.