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First identify what is failing: a missing folder, missing messages, a message body that will not load, or content that is unavailable offline. Those symptoms point to different checks. Compare Thunderbird with your provider’s webmail, then check folder subscriptions and per-folder synchronization before trying to repair a local folder copy.
Start by identifying what is missing
IMAP keeps mail on the server and synchronizes Thunderbird’s local view with server-side changes. A change made in one place, such as deleting a message, can be reflected in the other. Before changing settings, check whether the folder or message appears in your provider’s webmail.
- The folder is absent in Thunderbird: Check whether it is subscribed.
- The folder appears, but messages are missing: Check webmail, then consider whether Thunderbird’s local folder copy is stale.
- A message header appears, but the body will not load: Thunderbird may have the header without a locally stored message body.
- Messages appear online but are unavailable offline: Check the account’s synchronization settings and the selections for individual folders.
These distinctions matter because folder visibility and offline storage are separate. A folder can be visible without its contents being synchronized for offline use.
If an IMAP folder is missing, check its subscription
Thunderbird may not display every folder on an IMAP account if the folder is not subscribed. Open the account’s folder subscription list, refresh the list if that option is available, and subscribe to the folders you expect to see. Mozilla Support identifies subscriptions as a possible cause of missing folders: Thunderbird not downloading offline folders.
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If the folder is not present in webmail either, the problem may be with the server-side folder or account rather than Thunderbird’s local display. Check the provider’s guidance for that account.
If the folder appears but is not available offline, check synchronization
In Thunderbird, open the account’s Synchronization & Storage settings. Confirm that local message storage is enabled if you want offline access, then inspect the folder list and select each folder you need—including subfolders.
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Folder selection is per folder: Mozilla’s IMAP Synchronization documentation says, “Sub-folders are not automatically synchronized just because the parent folder is checked.” Selecting a parent folder therefore does not ensure that its subfolders have offline copies.
Synchronization can use local disk space. Exact labels and navigation can vary by Thunderbird version and operating system, so if you do not see these controls, consult the help for your installed version rather than assuming the setting is missing.
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Understand the difference between headers and message bodies
A message listed in Thunderbird does not necessarily have its full body stored on the computer. With limited synchronization, Thunderbird can show message headers and fetch a body when you open it. That body may not be available without an internet connection until it has been synchronized.
If the body will not load while you are online, check whether the message opens in webmail. If webmail can display it but Thunderbird cannot, the issue may be a local copy or connection problem; if webmail cannot display it either, check with the provider.
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Repair a local folder when webmail has the missing message
If you can verify that a message is still on the server in webmail but it is missing from Thunderbird’s local folder view, try repairing that folder:
- In Thunderbird’s folder pane, right-click the affected folder.
- Open Properties.
- Choose Repair Folder.
- Allow Thunderbird to refresh the folder, then check whether the message appears.
A Mozilla Support accepted answer recommends this action for a specific missing-message case: Thunderbird lost 2 messages syncing with IMAP server. Treat it as a targeted attempt to correct a stale or inconsistent local folder, not a guaranteed fix for every IMAP problem.
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If the mismatch continues, check the provider and connection
If the expected folders are subscribed and selected for synchronization, and repairing the affected folder does not resolve a verified message mismatch, investigate the account’s connection to the mail server. Check the provider’s service-status information, IMAP settings, authentication, and any error Thunderbird displays. The correct server details depend on the provider, so do not substitute generic or guessed settings.
Before broader mailbox or profile changes, verify important mail in webmail and preserve a backup. Since IMAP changes can propagate between Thunderbird and the server, avoid deleting or moving messages as a test unless you understand how the provider handles those changes.
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