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Problem guide · approved for publication

Thunderbird does not download IMAP mail for offline use

IMAP keeps mail on the provider and can download message bodies on demand; offline availability depends on synchronization settings and folder selection.

APPROVEDDOCUMENTED ONLYChecked 2026-10-15

Symptoms

What you may notice

  • A message list is visible but a message body is unavailable without a connection.
  • Only some folders are available offline.
  • A user moving from POP expects every message to be stored locally.

Possible causes

Why this can happen

  • The folder is not selected for offline use.
  • The account has not finished synchronizing or local disk space is limited.
  • The provider retains only a subset of messages or applies server-side policies.
  • The account is being migrated between POP and IMAP and local copies have not been backed up.

Diagnostics

A safe way to narrow it down

  1. 01

    Confirm whether the account is IMAP or POP and record the Thunderbird version and provider.

  2. 02

    Check the account synchronization/storage settings and which folders are selected for offline use.

  3. 03

    Wait for synchronization on a stable connection and verify a non-critical message offline.

  4. 04

    Check local disk space and provider retention before changing bulk settings.

  5. 05

    Before migrating an account, preserve a local backup and follow Mozilla's migration guidance.

Solutions

Documented approaches

Select the required folders for offline synchronization

DOCUMENTED

Use Thunderbird's synchronization and storage settings to select the folders that should be available offline. Test with a non-critical folder first.

Read supporting source

Back up before a POP-to-IMAP migration

DOCUMENTED

Keep a local copy and follow the official migration guidance. The email provider remains responsible for mailbox retention and server access.

Read supporting source

Alternatives and limits

Keep the trade-offs visible

Use provider webmail for server-only access

Webmail requires connectivity and does not create an offline copy; it can be useful to distinguish a local sync issue from a server issue.

Evidence

What this page is based on

Mozilla: IMAP synchronization

How IMAP synchronization and offline storage work.

Official source
Mozilla Support: offline download discussion

Offline-download context and troubleshooting questions.

Official source
Mozilla: switch from POP to IMAP

Migration considerations and backup boundary.

Official source

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