A large mailbox is easier to migrate when you treat it as a series of small, verifiable moves rather than one all-or-nothing task. Batches make it simpler to spot missing folders, unexpected count changes or older messages that did not transfer before you continue. The exact steps depend on the import tool and mail providers, so this guide focuses on checks that work across different setups. Keep the original mailbox available throughout, record what each batch covers, and avoid deleting or redirecting mail until you have reviewed the destination.
The useful bits
- Record folder names and message counts before moving anything.
- Check a small first batch, including representative older messages.
- Keep the source intact until destination checks are complete.
Prepare a map before you move mail
Start by listing the folders you expect to migrate, including custom folders and any folders that contain older or important correspondence. Record the approximate message count for each folder if your current mail system shows one. Counts may change as new mail arrives, and different systems may count conversations, drafts or deleted items differently, so note how you obtained them rather than treating them as an exact universal standard.
Decide what belongs in scope. For example, you might include Inbox, Sent, and named project folders, but leave Trash out unless you specifically need it. Check for duplicate or similarly named folders before importing; an import tool may combine them, preserve both, or handle them another way. Keep a simple checklist with the folder, planned batch, starting count and final check.
Folder: Project Archive Starting count: [count shown by source] Planned batch: [date range or subset] Destination check: [count and sample reviewed]
Check capacity and access first
Before transferring a large archive, confirm that the destination mailbox has enough available storage for the mail and attachments you plan to keep. BITTU Mail personal @bittumail.tech mailboxes start with 500 MB; Business Starter, Standard and Premium provide 10, 20 and 50 GB per mailbox respectively. Check the account’s actual available capacity and current plan rather than assuming the entire allowance is free.
Make sure you can sign in to both accounts and that the migration method you choose supports the source and destination. If connecting a mail app to BITTU Mail by IMAP, the published incoming server is mx1.bittuicloud.com on port 993 with SSL/TLS. Do not accept certificate warnings. Account actions go through mail.bittuicloud.com. Avoid starting a migration while access, storage or account setup is still uncertain.
Choose batches you can verify
Split the work by folder, date range or another clear boundary supported by your chosen tool. A batch should be small enough that you can check it before starting the next one. For instance, migrate one custom folder first, or divide a very large archive into date periods. Use the same boundary in your notes and in the tool so you can tell exactly what was included.
Begin with a representative pilot batch rather than the easiest-looking slice alone. Include ordinary messages, messages with attachments, and some older mail if the tool allows you to select it. Copying rather than moving can preserve the source during verification, but whether that option exists depends on the tool. Do not assume that a completed progress indicator means every folder and message arrived correctly.
Use the same checks after every batch
When a batch finishes, check that its destination folder exists and that the expected messages appear in it. Compare the destination count with the source count for that batch, allowing for known differences in how the two systems count conversations or excluded items. A mismatch is a reason to investigate, not proof by itself that mail is missing.
Open several messages from different points in the batch. Check the sender, recipient, subject, date, body and attachments where relevant. Include older mail in these spot checks: search for a known older subject or sender and confirm that the message opens with its expected date and content. If you cannot find a message, search the destination more broadly before concluding it failed; it may have landed in another folder or been grouped differently.
- Folder is present and named as expected.
- Message count is recorded and any difference has an explanation.
- Representative recent and older messages open with the expected details.
Pause when something does not match
If a folder is absent, counts differ unexpectedly or sampled messages are incomplete, stop before launching the next batch. Check the tool’s error report and selection settings, then compare the source and destination for the affected folder or date range. Look for skipped items, size limits, duplicate handling or filters, but rely on the tool’s actual documentation rather than assuming a particular cause.
Record what you found and retry only the affected portion when you understand the issue. Repeating a whole batch without checking duplicate behavior can create extra copies. If the tool’s report is unclear, preserve the source and seek help from the tool or provider responsible for that step.
Finish with a coverage review
After all planned batches, compare your original folder map with the destination. Mark each folder as checked, note any deliberate exclusions, and review counts for the folders that matter most. Search for representative older messages across the archive, not only the newest mail. If new messages arrived during the migration, identify how you will handle that gap before considering the move complete.
Keep the original mailbox available until you are satisfied with coverage and have a plan for any mail that arrived during the transfer. Once checks are complete, decide separately whether to change where new mail is delivered or retire the old account; those actions are not required to verify the migration itself. A written record of batches, exceptions and final checks makes it easier to resume safely if the process is interrupted.
About this guide
Prepared with AI assistance for the BITTU Mail editorial library. Check important details against your original messages and current account settings. Product access depends on your role, plan and provisioning state. Provider interfaces and requirements can change.
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