PST, OST, MBOX, EML and MSG explained

At a glance

PST: Outlook’s archive

A Personal Storage Table holds folders and items in one file, in a format Microsoft documents openly ([MS-PST]). It is the only copy of what it holds, so PST files are what people archive, back up and move between computers. Modern PSTs are Unicode files with a default limit of 50 GB; very old ANSI files stop at 2 GB.

OST: the offline copy

An Offline Storage Table uses the same internal structure as a PST, but it is a cache: the mailbox lives on the server, and Outlook keeps the OST in sync. It opens only in the profile that created it. When the account is gone (a former job, a deleted mailbox), the OST may be the last copy, and converting it to PST makes it readable again.

MBOX: a folder in one text file

An MBOX file stores one folder as plain text: each message starts with a line beginning From . Thunderbird keeps every folder this way, Apple Mail exports mailboxes as MBOX, and Google Takeout delivers Gmail as MBOX.

EML: one message, the Internet standard

An EML file is one message exactly as it travels between mail servers (RFC 5322 and MIME): headers, body and attachments as text. It is the most portable format, opens almost anywhere and is a good long-term archive.

MSG: one Outlook item

An MSG file holds one Outlook item with all its Outlook properties, including categories, flags and calendar or contact fields that EML cannot carry. Outlook creates one when you drag a message out of it.

Converting between them

Not covered yet: Outlook for Mac’s .olm export.