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Start by identifying what is missing: the folder, new messages, a message body, or an offline copy. Then check the matching Thunderbird setting and confirm the item still exists in your provider’s webmail. IMAP folders can be visible without being fully stored on your computer, so a missing offline copy does not necessarily mean the server has lost the message.
First, identify the kind of sync problem
IMAP keeps Thunderbird’s view of a mailbox in sync with the mail server. Changes made in one place can be reflected in the other, so verify important messages in webmail before making broader mailbox or profile changes. The Thunderbird IMAP Synchronization guide explains how local synchronization works.
- The folder is missing: Check whether it exists in webmail, then check Thunderbird’s folder subscriptions.
- The folder appears, but messages are missing: Check the server copy in webmail, then the folder’s synchronization settings.
- A message header appears, but its body will not load: A header alone does not mean the full message is stored locally; test whether Thunderbird can fetch the body while online.
- The message is available online but not offline: Check whether that folder and its messages are selected for local synchronization.
If the item is also missing from webmail, Thunderbird may not be the cause. Check with your provider before treating it as a local-copy problem.
If an IMAP folder is missing, check its subscription
Folder visibility and offline storage are separate settings. An IMAP folder that is not subscribed may not appear in Thunderbird, even if it exists on the server.
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- Open the account’s folder subscription list in Thunderbird. The exact menu label and location can vary by version and operating system.
- Refresh the list if Thunderbird offers that option, then look for the missing folder.
- Subscribe to the folder you want to see and allow Thunderbird to update the folder list.
- Check whether the folder now appears. If it does but its messages are not available offline, continue to the synchronization settings below.
Mozilla Support discusses subscription and settings checks in an example about Thunderbird not downloading offline folders. It is a forum case, not proof that subscription is the cause of every missing-folder problem.
If the folder appears but content is unavailable offline, check synchronization
Open the account’s Synchronization & Storage settings and review whether local message storage is enabled if you want messages available without an internet connection. Then inspect the individual folder selections and choose each folder—and each subfolder—you need offline.
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Thunderbird’s documentation is explicit: “Sub-folders are not automatically synchronized just because the parent folder is checked.” A selected parent folder therefore does not confirm that its children have local copies. See Mozilla’s IMAP Synchronization documentation for the settings and their effect.
Synchronization uses local disk space. If storage is limited, Thunderbird may show message headers while fetching a body only when needed. Such a body may not be available offline until it has been downloaded. To distinguish a download problem from an offline-copy setting, connect to the internet and try opening the message; then check it again offline.
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If one message is missing, compare Thunderbird with webmail
Search for the message in the provider’s webmail first. If it is there but absent from Thunderbird’s local folder view, the local folder copy may be stale or inconsistent.
- In Thunderbird, right-click the affected folder.
- Choose Properties.
- Choose Repair Folder, then let Thunderbird refresh the folder.
- Check whether the message reappears and compare the result with webmail.
A Mozilla Support accepted answer recommends this action for a specific missing-message case: Thunderbird lost 2 messages syncing with IMAP server. Repair Folder is a targeted attempt to refresh a folder’s local data, not a guaranteed remedy for every sync failure. Confirm that important messages remain on the server before trying it.
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If the built-in checks do not resolve it
When the folder is subscribed, its synchronization choices are correct, and a folder repair has not resolved a server-confirmed mismatch, investigate the account connection rather than repeating local steps.
- Check the provider’s service-status information for an outage or maintenance affecting mail.
- Review the provider’s current IMAP server and connection requirements against the account settings in Thunderbird.
- Look for authentication prompts or a specific connection error, and follow the provider’s guidance for resolving it.
- Before broader changes to a Thunderbird profile or mailbox, preserve a backup and verify important messages in webmail.
The correct server details and troubleshooting path depend on the provider and account. Without those details, there is no safe universal server name or setting to recommend. Exact Thunderbird labels can also differ by version and operating system.
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