Manual method
- Identify the provider and client, and download all permitted folders for offline use. Preserve any existing MBOX/EML/PST files without converting the only copy.
- For Thunderbird, use Tools, Export for a profile small enough for its ZIP exporter.
- For profiles above the documented 2 GB ZIP limit, locate the profile folder, close Thunderbird and copy the entire folder to a dated backup directory.
- Verify a copied profile in an isolated client profile or inspect portable message files. Keep the original untouched until recovery is demonstrated.
What you get: Provider authentication, server archives and folder subscriptions vary. A Thunderbird profile includes settings and potentially credentials as well as messages; encrypt and restrict it. For long-term portability, retain standard message exports alongside application-specific recovery data where the client supports them. Provider instructions. Supporting reference 1.
Size and waiting time
Planning example: 10 GB; light/heavy scenarios: 0.1–100 GB. These are estimates, not a measured population average or fixed limits. One or several legacy mailboxes with around 50,000 messages at 200 KB each; exported duplicates and attachments affect size.
Preparation: Initial IMAP collection can take hours or days under throttling. Copying a completed local profile is limited by disk speed, not internet speed.
Network transfer of 10 GB: about 3.2 hours at 10 Mbps, 38 min at 50 Mbps, or 19 min at 100 Mbps. This assumes 70% sustained throughput and excludes preparation, manual work and extraction. For local exports, this is a comparison for transferring the saved files online; disk speed determines local copy time. Online backups use your upload speed. Transfer calculations and large-file examples.
Store and check your archive
An encrypted external SSD or hard drive is a good main archive for this collection. Allow roughly 30 GB of working space for the example, including an original, an extracted working copy and room to check it. Very compressed packages can need more. A USB key is useful for a spare copy, but should not be the only one. An encrypted online copy protects against losing the local devices.
Keep the original in other_mail/YYYY-MM-DD/original/. Record the account alias, selected scope, date range and missing items. Open old and recent records and sample attachments offline, then compare file sizes or checksums after copying. Keep the source account until you have checked the result. Storage and recovery instructions.
Sources and review
Public documentation was reviewed on 27 October 2026. No signed-in export or restoration was performed. Account options and availability can change; follow the current provider instructions.