How to convert Gmail Takeout mbox to PST for Outlook

Outlook has no built-in mbox support. The two reliable conversion paths are: (1) import the mbox into Thunderbird, then export to PST via the ImportExportTools NG add-on; or (2) use a dedicated mbox-to-PST converter like Aid4Mail, Systools, or Stellar. Path (1) is free but slow on large archives; path (2) is faster but typically $40-$200.

Why Outlook can’t open mbox

Microsoft picked its own PST format in the 1990s and never added mbox support. Every path from Gmail Takeout to Outlook goes through a conversion. Expect minor loss on flags (read/unread state, stars → categories) and exact timestamps.

Option 1: Thunderbird as a bridge (free)

1. Install Thunderbird and the ImportExportTools NG add-on.
2. Create a new Local Folder in Thunderbird.
3. Right-click the folder → ImportExportTools NG → Import mbox file, select the Takeout .mbox.
4. Once imported, right-click → ImportExportTools NG → Export folder → PST (requires a working local Outlook profile).

Preserves MIME fidelity and costs nothing. Thunderbird struggles above ~10 GB, so plan on splitting very large archives by label first.

Option 2: Commercial converters

All three process archives over 50 GB. None are open source; none are free for commercial use.

What gets lost in conversion

• Gmail labels become Outlook categories, but only the primary label. Messages with multiple labels lose the extras.
• Stars become the “Yellow Category” in Outlook.
• Thread IDs are rebuilt from Subject and Message-ID headers. Conversations can split if headers are malformed.
• Attachments stay intact — PST supports the same MIME types mbox does.

Hash change matters for e-discovery

Converting mbox to PST changes file-level hashes because the container format differs. Always preserve the original .mbox alongside the converted .pst for chain-of-custody. Many discovery workflows require both: the original archive and the Outlook-readable copy.

Should you convert at all?

If the goal is to read, search, and archive the messages — not to use Outlook-specific features like calendar integration — a modern mbox viewer is faster, free, and skips the conversion entirely. tomorrow-box is built for exactly this case: opening a Gmail Takeout archive without shoe-horning it into PST.

Checklist

• Original .mbox file preserved with a verified checksum
• Conversion tool chosen (Thunderbird or commercial)
• Target .pst opens in Outlook and spot-checks against the original
• Labels / categories reviewed for mapping loss
• Both files stored per your retention policy