Email Box Migration
There are different ways to migrate an email box:
Migrate the box via the email client
Advantages:
- Data does not pass through third-party servers.
- Email accounts are created directly in the email client, such as Thunderbird / Outlook.
- No additional cost is incurred by a relocation service.
Disadvantages:
- The migration is done via your own internet connection, which can take time or fail depending on speed, stability, and box size.
Migrate the box via the Audriga relocation service
Advantages:
- The box does not need to be transferred via your own internet connection. Thus, even with a slow connection, large boxes can be reliably transferred.
- It is possible to transfer multiple boxes at once without configuration in the email client.
Disadvantages:
- The relocation service provider has access to your email box and therefore to your emails. This can pose compliance issues with your company’s privacy policy.
- Relatively high costs per box apply.
Migrate the box via the “ImapSync Online” relocation service
Advantages:
- No additional cost is incurred.
- The box must be transferred via your own internet connection. Even with a slow connection, large boxes can be reliably transferred.
- The program underlying this service, “ImapSync”, is open source.
Disadvantages:
- The relocation service provider has access to your email box and therefore to your emails. This can pose compliance issues with your company’s privacy policy.
- The user interface is confusing, as it directly displays the outputs of the ImapSync command line program.
Which migration method should you choose? (decision guide)
The “best” mailbox migration method depends on scale, time constraints, and the sensitivity of the mailbox content. Start by identifying your situation: how many mailboxes you need to move, their size, and whether you can use third-party tools under your internal privacy/compliance rules.
Use this decision guide:
- Choose an email client migration (Thunderbird/Outlook) if you are moving one or a few smaller mailboxes, you want to avoid third-party access, and you can keep the client running reliably throughout the copy. This method is often simple but can be slow and may fail if the local connection drops.
- Choose a specialized migration service (such as Audriga) if you are moving many mailboxes, huge mailboxes, or you need a process that does not depend on a single workstation staying online. This is typically the most scalable approach, but it incurs costs and requires third-party access to mailbox contents.
- Choose an online IMAP sync tool (such as an IMAPSync-based service) if you want a low-cost option and can accept a more technical interface. This can work well for straightforward migrations but still involves third-party access and requires careful configuration.
If you are unsure, prioritize reliability and controlled cutover. The migration method should support your primary goal: moving historical mail accurately while ensuring new mail continues.
Before you start: prerequisites and information you need
Most mailbox migrations fail due to missing credentials, wrong server settings, or insufficient destination storage. Prepare the following items before starting any migration method:
- Source mailbox access: full email address (or username) and password for the old mailbox, with IMAP enabled.
- Destination mailbox access: full email address (or username) and password for the new mailbox, with IMAP enabled.
- Server details: incoming (IMAP) server name, port, and encryption method for both source and destination.
- Capacity check: Confirm the destination mailbox has enough quota to hold the migrated messages and attachments. Add buffer for growth and duplicates.
- Folder expectations: confirm which folders must be migrated (Inbox, Sent, Drafts, custom folders). Decide whether to migrate spam/trash folders or leave them behind.
- Timing plan: choose a migration window and determine whether the migration will run in a single pass or in multiple passes (an initial bulk copy followed by a final sync).
Having these basics ready prevents repeated failures and reduces downtime during the move.
Avoiding mail loss during transition: forwarding and dual-delivery strategy
A mailbox migration has two streams: historical mail being copied and new mail still arriving. The most significant risk is losing emails sent during the transition window. The safest approach is to ensure that incoming mail is delivered reliably while the copy runs.
Recommended approach:
- Run an initial migration pass first while the old mailbox is still active. This copies the bulk of historical mail.
- Enable a temporary forwarding or dual-delivery strategy so that new inbound mail is not missed. The best method depends on your setup:
- If the old mailbox still receives mail, forward new messages to the new mailbox during the migration window.
- If you control mail routing at the domain level, plan the cutover so you can switch delivery to the new mailbox only after the new mailbox is ready.
- Perform a final sync pass shortly before cutover (or immediately after) to copy messages that arrived during the initial pass.
Avoid switching MX records too early if the destination mailbox is not fully prepared. A controlled dual-delivery period is the most reliable way to prevent “missing emails” complaints after the move.
Validation checklist after migration
Do not consider the migration complete until you validate content and ongoing mail flow. A structured validation step prevents silent data loss and reduces post-migration support work.
Use this checklist:
- Login and access: confirm you can log in to the destination mailbox via webmail and/or mail client.
- Folder coverage: verify that all required folders exist and contain expected messages (Inbox, Sent, and any custom folders).
- Spot-check recent mail: compare a sample of recent emails in the source and destination, including messages with attachments.
- Message counts (sanity check): compare approximate message volume per key folder. Exact equality is not always required, but large discrepancies require investigation.
- Search and attachments: confirm you can search for older mail and open attachments, which helps validate that messages transferred intact.
- New mail delivery: send a test email from an external address to confirm the destination mailbox receives it, and send outbound mail to confirm sending works.
If any checks fail, do not proceed with final cutover actions. Re-run synchronization or adjust the migration method until the validation passes.