Email Migration Checklist: Everything You Need Before Switching

Before You Migrate: What to Evaluate

Moving thousands of messages between platforms involves format conversions, metadata preservation, folder structure mapping, and edge cases that only surface after you have committed to the process.

A structured approach prevents data loss, reduces downtime, and avoids the frustration of discovering problems after the old system has been decommissioned. This checklist walks through every stage of an email migration, from initial planning to final verification.

Whether you are moving from Thunderbird to Outlook, switching away from Gmail, or consolidating Apple Mail archives, the fundamental steps are the same. Read through this entire checklist before starting to estimate the time required and identify potential blockers early.

Step 1: Inventory Your Email Data

Before touching any conversion tool, you need a clear picture of what you are migrating. Open your current email client and document the following.

Total mailbox size. Check how much data you are working with. This determines whether you can migrate everything at once or need to work in batches.

Number of accounts. If you manage multiple email accounts in a single client, list each one. Each account may need to be exported and converted separately.

Folder structure. Document your folder hierarchy, especially nested folders more than two or three levels deep. Some conversion tools flatten deep structures.

Special data types. Note whether your mailbox contains calendar events, contacts, tasks, or rules/filters. Calendar and contacts often require separate export steps.

Attachment volume. Attachments over 25 MB increase processing time significantly. Note them for special handling.

Date range. Know the oldest and newest messages in your archive to verify completeness after migration.

Step 2: Identify Your Source and Target Formats

Every email client stores data in a specific format. The table below maps the most common clients to their native formats.

Email client	Native format	File extension
Microsoft Outlook (Windows)	PST	.pst
Microsoft Outlook (cached)	OST	.ost
Mozilla Thunderbird	MBOX	.mbox (or no extension)
Apple Mail	EMLX	.emlx
Gmail (via Google Takeout)	MBOX	.mbox
Generic / portable	EML	.eml

Your migration path is determined by the combination of source and target formats. For example, moving from Thunderbird to Outlook means converting MBOX to PST. Moving from Outlook to Apple Mail means converting PST to EMLX or MBOX.

If your source client is not listed here, check whether it supports exporting to EML format. EML is the most universal email interchange format and can be converted to any other format.

Step 3: Choose a Conversion Method

With your source and target formats identified, select the appropriate conversion approach.

Direct format conversion is the simplest path when a tool supports your specific source-to-target combination. MailtoPst supports conversions between PST, MBOX, EML, EMLX, MSG, and OST formats. Upload your source file, select the target format, and download the result.

Two-step conversion may be necessary for less common format pairs. For example, if no direct converter exists for your specific combination, you can convert to EML as an intermediate format. EML serves as a universal bridge because every major email client can read and write it.

Manual migration through IMAP is an alternative when both your source and target clients support IMAP accounts. Set up both clients to connect to the same IMAP server, then drag messages between accounts. This approach is slow for large mailboxes but avoids file format conversion entirely.

For most migration paths, direct format conversion offers the best balance of speed, reliability, and metadata preservation.

Step 4: Back Up Everything

This step is non-negotiable. Before you convert, export, or modify anything, create a complete backup of your current email data.

Export from your email client. Use your client’s built-in export function to create a local copy of your entire mailbox. In Outlook, export to PST. In Thunderbird, copy your profile directory. In Apple Mail, use the Export Mailbox function.

Copy the backup to a separate location. Do not keep your only backup on the same drive as the original data. Copy it to an external drive, a different computer, or cloud storage. The point is redundancy.

Verify the backup. Open the backup file in a separate client instance or on a different machine to confirm it contains all your data. A backup you have not verified is not a backup.

Keep the backup until the migration is fully verified. Do not delete your backup until you have completed every step in this checklist, including the final verification phase. Weeks, not hours.

Step 5: Run a Test Conversion

Never run a full migration on your first attempt. Start with a test using a small, representative subset of your data.

Select a test sample. Choose a folder with 50 to 100 messages that includes a variety of content: plain text emails, HTML-formatted messages, emails with attachments of different sizes and types, messages with inline images, and messages in different languages if applicable.

Run the conversion. Process your test sample through your chosen conversion method and import the result into your target email client.

Verify the output. Check the following for each test message. Does the sender and recipient information match the original? Are dates and times preserved correctly, including time zones? Do attachments open without errors? Is the HTML formatting intact, including fonts, colors, and layouts? Are read/unread flags preserved? Is the folder structure maintained?

Document any issues. If something does not convert correctly, investigate before proceeding. Common issues include character encoding problems with non-Latin scripts, attachment filename corruption, and loss of message flags or categories.

MailtoPst provides 100 MB free credit, which is designed precisely for this test phase. Use it to validate the conversion quality before committing to a full migration.

Step 6: Execute the Full Migration

With a successful test behind you, proceed to the full migration.

Plan for downtime. Decide whether you will continue using your old email client during the migration or switch entirely. If you continue receiving new emails during the migration, you will need a follow-up pass to capture messages that arrived after the initial export.

Process in manageable batches. For large mailboxes (over 5 GB), consider converting in batches organized by year or by top-level folder. This makes it easier to track progress, resume if something goes wrong, and keep individual output files within size limits recommended by your target client.

Monitor the conversion. Watch for error messages or warnings during processing. A conversion tool that silently skips problematic messages can leave gaps in your migrated data.

Import into the target client. Follow your target client’s import procedure. For Outlook, use File > Open & Export > Import/Export. For Thunderbird, use the ImportExportTools NG add-on. For Apple Mail, use File > Import Mailboxes.

Record timestamps. Note when you started and completed the migration. This creates a clear boundary for identifying messages that might have been missed.

Step 7: Verify and Clean Up

The migration is not complete until you have verified the result thoroughly.

Compare message counts. Small discrepancies (under 1%) may be acceptable if they correspond to duplicates filtered during conversion. Larger discrepancies warrant investigation.

Spot-check across the date range. Open messages from different time periods: the oldest, the most recent, and several points in between. Verify content, formatting, and attachments.

Test search functionality. Run several searches to confirm that the search index covers the imported messages. Some clients need time to rebuild their index after large imports.

Verify special data. Check calendar events, contacts, and tasks migrated through separate exports. Recurring events and task due dates are common failure points.

Update rules, filters, and signatures. These do not transfer between clients. Recreate them manually in the target application.

Keep both systems running in parallel. Maintain access to your old email system for at least two weeks. This gives you a fallback if issues surface later.

Common Migration Paths

Here are quick links to detailed guides for the most frequently requested migration scenarios.

Thunderbird to Outlook

The most common corporate migration path. Convert MBOX files to PST format and import into Outlook. Our Thunderbird to Outlook migration guide provides a step-by-step walkthrough.

Outlook to Thunderbird

Popular among users moving to open-source platforms. Export your Outlook mailbox as PST, convert to MBOX, and import into Thunderbird. See the Outlook to Thunderbird migration guide for details.

Gmail to Outlook

Use Google Takeout to export your Gmail as MBOX, convert to PST, and import into Outlook. The Gmail to Outlook migration guide covers the full process including contacts and calendar.

Apple Mail to Outlook

Export from Apple Mail as EMLX files, convert to PST, and import into Outlook. The Apple Mail to Outlook migration guide addresses the specific challenges of this path, including handling of Apple Mail’s unique EMLX format.

For a comprehensive overview of all supported migration paths, visit our email migration use cases page.

For guidance on preserving your email data for the long term regardless of which client you use, see our email archiving best practices guide.

Start Your Migration

A successful email migration comes down to preparation. Inventory your data, identify your formats, test before you commit, and verify after you finish. Rushing through any of these steps risks data loss or a migration that needs to be repeated.

MailtoPst supports conversions between all major email formats, including PST, MBOX, EML, and EMLX. Whatever your source and target platforms, the conversion step is handled. Start with 100 MB free credit to test your specific migration path.

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