PST to MBOX: what you get, measured on 650 emails
MBOX is one text file with every message one after another, each starting with a line that begins with “From ”. We exported a 650-message archive (1,541,120 bytes) with the Mailattic engine and compared it with the same messages as separate EML files.
| Measure | Value |
|---|---|
| Messages in the PST | 650 |
| Lines starting “From ” in the MBOX | 650 (one per message) |
| Separate EML files, total bytes | 516,971 |
| One MBOX file, bytes | 536,445 |
| Extra bytes in MBOX | 19,474 (about 30 bytes per message) |
| Export time on the test PC | 40 ms for all 650 (EML and MBOX) |
Details
Why the file is larger: each message gets a separator line (sender and date) and a blank line. A body line that itself starts with “From ” gets a “>” in front, otherwise a mail program would think a new message begins there.
Measured on one generated archive of plain-text messages with no attachments. Thunderbird and Apple Mail import MBOX; check the import in your own program. Time is one run on a Linux PC. Tested 2026-10-31. mailattic-core 1.0.0.
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About Mailattic
Mailattic is our Windows app that opens a .pst without Outlook, reads attachments, and exports EML, MBOX and PDF. Read-only: your archive is never changed.
Not affiliated with, endorsed by or connected to Microsoft. Outlook and Microsoft are trademarks of Microsoft Corporation and are used only to say which files this opens.