Understanding PST and OST Mailbox Archive Format & Differences.

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@Understanding OST and PST Files -

Two Primary file type in Microsoft Outlook- PST and OST. While both formats serve essential roles, they function differently depending on your account type and usage needs.

What are OST Files?

OST files are local copies of mailbox data from Exchange, Outlook.com, or Office 365 accounts. They allow users to access and work with emails, calendars, contacts, and tasks offline. Once the internet connection is restored, all changes are automatically synced with the server.

Key Features of OST Files:

  • Offline Access: Continue working even without an internet connection.
  • Synchronization: Updates are synced to the server once online.
  • Recovery Friendly: If the server fails, data can often be restored from the OST file by re-syncing.

 

What are PST Files?

PST files are data files used primarily with POP3, IMAP, or web-based email accounts. They store user data locally and are often used for personal backups, archiving, or transferring Outlook data between systems.

Key Features of PST Files:

  • Personal Backup: Ideal for storing personal folders or old emails.
  • Portability: Easily copied or transferred across devices.
  • File Size Limitations: Large PSTs can slow down Outlook performance—monitoring file size is important.


@Basic Difference Between PST and OST Archive Format: 

Format

PST File

OST File

Full Form

Personal Storage Table

Offline Storage Table

Usage

Manual backup or archive of mailbox data

Offline cache of mailbox for Exchange/Outlook

Created By

User/export tool/manual backup (POP3, IMAP, Webmail)

Outlook (when connected to Exchange, M365, or IMAP)

Storage Location

Local disk (user-defined)

Local disk (auto created by Outlook)

Primary Purpose

Long-term email storage, backup, or transfer

Enables offline access to mailbox

Mailbox Dependency

Can be imported into any Outlook profile

Tied to specific mailbox & profile

Sync with Server

❌ Not synced with Exchange or Microsoft 365

✅ Synced with mailbox on server

Archive Suitability

✅ Used as manual archive (e.g., exporting old mail)

❌ Not designed for archiving

File Location (default)

Documents\Outlook Files

AppData\Local\Microsoft\Outlook

Format Limit

Up to 50 GB (default), can be increased

Varies, usually 50 GB limit


@When we are used the PST & OST Format files -

Use PST Files -

  • 📦 Exporting old emails from Exchange/Outlook
  • 💾 Creating a local backup of a mailbox
  • 🔄 Moving mailbox data between accounts or computers
  • 📨 Archiving old emails not needed daily

Created via Outlook:
File > Open & Export > Import/Export > Export to a file > Outlook Data File (.pst)

Use OST Files -

  • 🔁 Cached Exchange Mode in Outlook (used for performance & offline use)

  • 🔗 Automatically created when you add an Exchange, Microsoft 365, or IMAP account to Outlook

⚠️ You can’t import or open an OST file manually – it only works with the account it was created from.


@How to Archive Mailbox at Microsoft 365

  • Step-01: Check the Prerequisite for Archive -
    • Microsoft 365 E3 / E5
    • Office 365 E3 / E5
    • Exchange Online Plan 2
    • Add-on: Exchange Online Archiving (EOA) for Plan 1

  • Step-02: Enable Archive Mailbox (Use Microsoft 365 Compliance Center)
    • Go to Microsoft Purview Compliance Center: https://compliance.microsoft.com
    • Navigate to: Data lifecycle > Archive
    • Select the user(s), then click "Enable Archive".
    • or///////
    • Use PowerShell to Enable Archive-
      • Enable-Mailbox -Identity user@domain.com -Archive
      • Get-Mailbox -Filter {ArchiveStatus -Eq "None" -and RecipientTypeDetails -eq "UserMailbox"} | Enable-Mailbox -Archive

  • Step-03: Set Archive Policies (Optional but Wise). Use Retention Tags and Policies to automate moving old emails to archive. For Example, Move emails older than 2 years to the archive.
    • Go to Microsoft 365 Compliance Center > Data lifecycle > MRM Retention Policies.
    • Create a Retention Tag > Action: Move to Archive > Age: e.g., 730 days.
    • Add the tag to a Retention Policy.
    • Apply the policy to a mailbox or group:  Takes up to 7 days to take effect.

  • Step-04: Verify Archive is Enabled-
  • Method-01: Sign in to the Exchange admin center (EAC) and navigate to Recipients > Mailboxes.
    • In the list of mailboxes, select the user to enable their mailbox for archive.
    • In the flyout pane, select Others, and under Mailbox archive, select Manage mailbox archive:
    • On the Manage mailbox archive pane, turn on Mailbox archive, and then Save.
    • \\\\\\\\\\\\\\ OR //////////////
  • Method-02: Verify Archive is Enabled
    • GUI: Go to Microsoft 365 Admin Center > Active Users
    • Click a User → Mail tab → Confirm In-Place Archive is active. OR,
    • Powershell: Get-Mailbox -Identity user@domain.com | FL ArchiveStatus,ArchiveName

Knowledge sharing@ahmizanpro

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