Step-by-Step PST 2 MBOX Conversion for macOS and Windows

PST 2 MBOX Best Practices: Preserve Attachments & Folder StructureConverting mail data from PST (Outlook Personal Storage Table) to MBOX (a common mailbox format used by Thunderbird, Apple Mail, and many other clients) is a frequent requirement when migrating between email clients or platforms. A successful PST to MBOX migration preserves message integrity, attachments, metadata (dates, sender/recipient), and the original folder hierarchy. This article explains best practices, common pitfalls, step-by-step guidance for both manual and tool-based approaches, and verification techniques to ensure attachments and folder structure remain intact.

Why preserving attachments and folder structure matters

Attachments often contain critical documents, images, or legal evidence. Losing them can break workflows or create compliance issues.
Folder structure reflects how users organize email (projects, clients, timelines). Preserving it minimizes user disruption and reduces time spent reorganizing after migration.
Metadata (timestamps, read/unread status, flags) maintains context and auditability of communications.

Pre-migration checklist

Inventory and sizing
- Check the total PST size and number of messages. Large PSTs (>10–20 GB) are more error-prone and may benefit from splitting before conversion.
- Note any corrupted PSTs (Outlook’s Inbox Repair Tool, scanpst.exe, can detect/fix many issues).
Backup
- Always create a complete backup of the PST file before attempting any conversion. Work on copies only.
Environment and client compatibility
- Identify target MBOX-compatible client(s) (Thunderbird, Apple Mail, Evolution, etc.) and any client-specific limitations (label vs. folder mapping).
- Ensure you have sufficient disk space for temporary files and converted mailboxes.
Account for special data
- Calendars, contacts, tasks, and notes in PST are not natively stored in MBOX. Plan separate export/import steps if you need to migrate those items.

Conversion approaches: pros and cons

Approach	Pros	Cons
Manual export/import (using Outlook + intermediary EML/EMLX)	No third-party tools required; greater control over selected items	Time-consuming; risk of losing metadata; tricky for large PSTs
Automated converter tools (commercial/free)	Fast; preserve metadata and attachments better; handle large files	Cost for reliable commercial tools; risk with unknown freeware
Using IMAP sync (upload PST mail to Outlook/Exchange, then sync to IMAP client and download MBOX)	Preserves structure and attachments; good for gradual migration	Requires server space; can be slow; depends on IMAP server behavior
Scripting with libraries (Python, readpst/mailbox)	Flexible and automatable; reproducible	Requires technical skill; may need format-specific handling for attachments/encoding

Recommended best practices by approach

Automated converter tools

Choose reputable tools with clear feature lists stating attachment preservation, folder hierarchy retention, and metadata support. Look for recent updates and good reviews.
Run a small test export (a few folders) and verify each message’s attachment, timestamp, and folder path in the resulting MBOX.
Use tools that support logging so you can audit conversions and re-run only failed items.
Prefer tools that handle large PSTs and Unicode correctly to avoid corrupted filenames or message text.

IMAP-based migration

Create a temporary IMAP account (or use an existing Exchange account) and add it to Outlook.
Move or copy PST folders into that IMAP account’s mailbox in Outlook; allow full synchronization.
Configure an MBOX-capable client (e.g., Thunderbird) with the same IMAP account and allow it to download messages; then use the client’s export/add-on to save mailboxes as MBOX.
Verify that folder names with special characters and nested folders replicate properly; some IMAP servers modify folder names (e.g., replacing slashes).

Manual export via EML/EMLX

Export messages as individual .eml/.emlx files retaining attachments. Most mail clients can import these into target clients which then can save MBOX.
Beware: this approach can lose or alter some metadata like original folder path unless you replicate folders manually.

Scripting and open-source utilities

Tools like readpst (part of libpst) can convert PST to mbox; run with flags that preserve attachments and message dates. Example: readpst -r -o output_dir file.pst (verify current flags in your installed version).
When scripting, ensure correct handling of character encodings (UTF-8) and attachment multipart boundaries. Test on diverse message types (HTML emails, calendar invites, signed/encrypted mails).

Handling attachments safely

Confirm attachments are embedded and not just links to cloud storage. For linked/cloud attachments, plan to download and reattach or preserve link metadata.
Watch for filename collisions when multiple attachments in different messages share the same name — reliable converters keep attachments inside their messages, avoiding collisions.
Verify that inline images (CID attachments) display correctly in the target client — some conversions may transform inline images into separate attachments or break references.

Preserving folder hierarchy and special folder mapping

Understand mapping differences: Outlook has folders like “Archive,” “Personal Folders,” and special folders (Sent Items, Deleted Items). Target clients may treat these differently or use labels instead.
Test mapping rules before full migration. For example, Thunderbird uses “Sent” and “Trash”; some tools map Outlook’s “Sent Items” -> “Sent” automatically, others place them in a custom folder.
Retain subfolder paths by using tools that export nested folder structure into separate MBOX files or into a single MBOX per folder with an accompanying folder tree file.

Dealing with common issues

Corrupt PST: run scanpst.exe to repair; if unrecoverable, try dedicated PST repair utilities before conversion.
Large single PST: split into smaller PSTs (Outlook allows creating new PSTs and moving folders) to reduce conversion failures and memory pressure.
Message truncation: occurs when converters fail on huge attachments or malformed MIME. Test and use converters with robust MIME handling.
Character encoding problems: use tools that explicitly support Unicode; verify messages containing non-Latin characters.

Verification and post-migration validation

Spot-check content: open a representative sample of messages from each folder — check body text, attachments, dates, senders/recipients, and flags (read/unread).
Folder structure: confirm folder names, nesting, and special folder mappings match expectations.
Message counts: compare message counts per folder between PST and MBOX exports.
Automated integrity checks: some tools produce logs or checksums; use them to identify missing items. For large migrations, create a checklist and sample plan (e.g., verify 10% of folders, 20 messages per folder).
User sign-off: have end users verify their most important folders and messages before decommissioning the old PST.

Performance and scalability tips

Convert during off-hours to avoid network and machine contention.
For enterprise migrations, use batch processing and automation; parallelize conversions across machines if possible.
Monitor disk I/O and temporary file locations; conversions can create large intermediate files.

Security and compliance

Treat PST files as sensitive data. Encrypt backups and delete temporary files securely after migration.
Ensure the migration process complies with retention and e-discovery policies. Keep an auditable trail (logs, test reports) of the migration.

Example quick workflow (recommended for reliability)

Backup original PST.
Run scanpst.exe and repair if needed.
Use a reputable converter or readpst to export PST to per-folder MBOXs in a test folder.
Import resulting MBOX files into the target client (Thunderbird/Apple Mail).
Verify attachments, message metadata, and folder structure on a sample set.
Run a full verification comparing counts and a final user review.
Securely archive original PST and conversion logs.

Final notes

A reliable PST-to-MBOX migration balances careful preparation, choosing the right tools, and thorough verification. Prioritize testing with representative data, ensure attachments and metadata are preserved, and keep users informed so the transition is smooth. When in doubt, run small pilots and iterate on conversion settings until results meet acceptance criteria.