PST, OST, MSG and EML: what each email file is
A .pst and an .ost are Outlook data files kept on your PC, a .msg holds individual items such as email messages, and an .eml holds one email message as plain text. MailCrate opens .eml, .msg and .pst files on Windows, and .ost files on a best-effort basis.
The four formats at a glance
| Format | What it holds | Where it comes from |
|---|---|---|
| .pst | Backed-up messages or older items kept on your PC; for a POP account, all Outlook information | Outlook data file |
| .ost | The offline folder file used with Cached Exchange Mode | Outlook data file |
| .msg | Individual email messages, appointments, contacts, tasks and similar items | Microsoft's MS-OXMSG format |
| .eml | One email message per file, as plain text | Internet Message Format, created to comply with RFC 5322 (see the Library of Congress entry) |
PST vs OST
A .pst backs up messages or keeps older items on your PC. For a POP account, all Outlook information lives in an Outlook data file. Microsoft explains this in its introduction to Outlook data files.
An .ost is the offline folder file used with Cached Exchange Mode.
Microsoft says the same settings govern the size of PST and OST files, in its article on size limits for Outlook data files. The default maximum size of a Unicode .pst or .ost is 50 GB in Outlook 2010 through 2016 and 20 GB in Outlook 2003 and 2007. Older ANSI files are limited to about 2 GB.
MSG vs EML
Microsoft's MS-OXMSG specification describes the .msg file format as the one used to represent individual email messages, appointments, contacts, tasks and similar items in the file system.
An .eml file holds one message in Internet Message Format, as plain text, and was created to comply with RFC 5322. The Library of Congress knows of no specification that defines EML as a file format. Outlook and other email programs open it, and a text editor shows it.
This is an example of the header lines at the top of an .eml file, using made-up addresses:
From: alex@example.com
To: sam@example.com
Subject: Lunch on Friday
Date: Fri, 23 Oct 2026 09:15:00 +0000
MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"So the practical difference is that an .eml is plain text you can read in a text editor, while a .msg can hold appointments, contacts and tasks as well as messages.
Opening a PST in the new Outlook
As of October 2026, Microsoft says the new Outlook can open a .pst only when classic Outlook is also installed (same bitness), with a Microsoft 365 subscription, and not on Arm devices. Microsoft also says it plans to remove the classic Outlook requirement.
That wording is on the same Microsoft support page. Check it for the current requirements before you plan around them, because they may change.
How to open these files without Outlook
MailCrate: open .eml, .msg, .pst and .ost files
MailCrate opens .eml, .msg, .pst and .ost files on Windows. Support for .ost is best-effort. For a .pst it shows the folder tree and search. Remote images are blocked by default, and you can allow them for one message at a time. It saves attachments and exports the text of a message. Your files are read on your PC and nothing is uploaded.
Frequently asked questions
What is the difference between PST and OST?
A .pst backs up messages or keeps older items on your PC. An .ost is the offline folder file used with Cached Exchange Mode. Microsoft says the same size settings govern both.
What is the difference between EML and MSG?
An .eml holds one message in Internet Message Format, as plain text, and a text editor shows it. A .msg is the Microsoft MS-OXMSG format for individual email messages, appointments, contacts, tasks and similar items.
Can I open a PST file in the new Outlook?
As of October 2026, Microsoft says the new Outlook can open a .pst only when classic Outlook is also installed (same bitness), with a Microsoft 365 subscription, and not on Arm devices. Microsoft also says it plans to remove the classic Outlook requirement.
Can I open a .msg file without Outlook?
Yes. MailCrate opens .msg files on Windows, along with .eml and .pst files and, on a best-effort basis, .ost files, and it exports the text of a message.
How big can a PST file get?
Microsoft says the default maximum size of a Unicode .pst or .ost is 50 GB in Outlook 2010 through 2016 and 20 GB in Outlook 2003 and 2007. Older ANSI files are limited to about 2 GB.