MailtoPst
FR
OST IMAP

Convert OST to IMAP
Online Tool

Recover and migrate Outlook OST emails to any IMAP server. Convert orphaned OST cache files to EML or MBOX, then upload to your IMAP mailbox. No Outlook needed.

Convert now → See how it works
🛡️ GDPR Compliant 🔒 TLS 1.3 Encryption 🇪🇺 Hosted in EU 🇫🇷 Built in France 🗑️ Auto-delete 24h

What is a OST file?

OST (Offline Storage Table) is a local cache file that Microsoft Outlook creates to store a synchronized copy of your Exchange or Office 365 mailbox. OST files allow you to work with your email offline, with changes synced back to the server when connectivity is restored.

OST files present unique challenges for migration:
- Profile-locked — an OST file can only be opened by the Outlook profile that created it
- Not portable — copying an OST to another computer does not make it readable
- Orphaning risk — if the Exchange account is deleted or the server decommissioned, the OST becomes inaccessible through Outlook
- No direct import path — IMAP servers cannot read OST files, and most migration tools ignore them

To migrate OST data to an IMAP server, you must first extract the emails into a standard format (EML or MBOX) using a specialized conversion tool like MailtoPst.

What is a IMAP file?

IMAP (Internet Message Access Protocol) is the universal protocol for accessing email on remote servers. Unlike the proprietary OST format locked to Microsoft Outlook, IMAP is supported by every major email client and server platform.

Migrating OST data to an IMAP server provides:
- Universal client support — Thunderbird, Apple Mail, Outlook, mobile apps, and webmail all use IMAP
- Multi-device sync — emails, read status, and folders stay synchronized across all devices
- Provider flexibility — switch between IMAP providers without losing access to your email
- Server-side storage — emails are safely stored on the server, not locked in a local cache file
- Standard protocol — IMAP (RFC 3501) is supported by every email system worldwide

Whether you use Gmail, Fastmail, Zimbra, Dovecot, or any other IMAP-compatible service, your converted OST emails integrate seamlessly.

Why convert OST to IMAP?

There are several important reasons to migrate OST data to an IMAP server:

1. Recovering email from orphaned OST files
When an Exchange server is decommissioned or an employee's Outlook profile is deleted, the OST file becomes the only copy of their email history. Extracting and uploading to an IMAP server recovers this data.

2. Transitioning away from Microsoft Exchange
Organizations moving from Exchange to alternative platforms (Zimbra, Postfix/Dovecot, or hosted IMAP) need to migrate both server-side and cached email data including OST files.

3. Consolidating scattered email data
IT departments often find OST files on decommissioned laptops, backup drives, and archived workstations. Importing this data to a central IMAP server organizes and makes it searchable.

4. Creating accessible archives
OST files locked to a single Outlook profile are useless for long-term archival. Uploading the extracted emails to an IMAP server creates an archive accessible from any client.

5. Litigation and eDiscovery
Legal teams may need email data from OST files accessible in a searchable, reviewable format. An IMAP server provides web-based access for legal review.

OST vs IMAP — Key Differences

Feature OST IMAP
Access model Profile-locked local file Username/password from any client
Portability Tied to one Outlook installation Accessible from any device
Protocol Proprietary Microsoft format Open standard (RFC 3501)
Multi-device Single device only All devices sync in real-time
Recovery Requires specialized tools Standard backup/restore procedures
Dependency Requires Exchange/Office 365 account Any IMAP provider

3 methods to convert OST to IMAP

Method 1: Convert OST to EML with MailtoPst, import via IMAP client

Upload your OST file to MailtoPst and convert to EML format. Download the EML files, then use an IMAP client like Thunderbird connected to your IMAP server to import the emails by dragging them into the appropriate mailbox folders.

Pros

  • ✓ Works with any IMAP server
  • ✓ No Outlook required
  • ✓ Selective import possible
  • ✓ Works on Mac, Linux, Windows

Cons

  • ✗ Client-side upload can be slow
  • ✗ Manual folder mapping
  • ✗ Requires IMAP client setup

Method 2: Convert OST to MBOX, then server-side import

Convert the OST to MBOX format using MailtoPst. Transfer the MBOX files to the IMAP server and use server administration tools (doveadm, mb2md, or similar) to import the MBOX files directly into user mailboxes.

Pros

  • ✓ Fastest for large datasets
  • ✓ No IMAP sync overhead
  • ✓ Batch import possible
  • ✓ Preserves folder structure

Cons

  • ✗ Requires server admin access
  • ✗ Technical knowledge needed
  • ✗ Server-specific commands
Recommended

Method 3: Convert OST to PST, then use Outlook IMAP sync

Convert the OST to PST with MailtoPst. Open the PST in Outlook, configure an IMAP account in Outlook, and move or copy emails from the PST folders to the IMAP mailbox folders. Outlook handles the IMAP upload automatically.

Pros

  • ✓ Familiar Outlook interface
  • ✓ Drag-and-drop simplicity
  • ✓ Preserves metadata

Cons

  • ✗ Requires Microsoft Outlook (Windows)
  • ✗ Slow for large archives
  • ✗ Paid software needed
Try MailtoPst →

Step by step

How to convert OST to IMAP with MailtoPst

01

Upload your OST file

Drag & drop or click to upload your .ost file. Files up to 51 GB supported with resumable upload.

02

Choose IMAP as output

Select IMAP from the format dropdown. MailtoPst auto-detects the best conversion settings.

03

Download your IMAP file

Conversion takes seconds. Download your .imap file or receive it by email.

Convert your OST to IMAP now →

100 MB free credit. No credit card required.

FAQ

Frequently Asked Questions

Can I import an OST file directly to my IMAP server?

No. OST files are in a proprietary Microsoft format that IMAP servers cannot read. You must convert the OST to EML or MBOX using MailtoPst, then import the converted files to your IMAP server.

My Exchange server no longer exists. Can I still recover my OST?

Yes. MailtoPst can read orphaned OST files directly without the original Exchange server. Upload the OST file from any browser, convert it, and import the output to any IMAP server.

Which IMAP servers work with this process?

All of them. Dovecot, Cyrus, Courier, Zimbra, Gmail, Fastmail, Yahoo Mail, and any other IMAP-compatible server will accept the converted email data. EML and MBOX are universal email formats.

Will email timestamps and metadata be preserved?

Yes. MailtoPst preserves all email headers including original send/receive dates, sender and recipient information, read/unread status, and importance flags. Your IMAP server will display the correct timestamps.

How long does it take to migrate a large OST file?

The MailtoPst conversion takes minutes. The IMAP import speed depends on your server and connection — expect 1-2 GB per hour via client-side IMAP upload. Server-side MBOX import is much faster, typically handling 10+ GB per hour.

Related conversions

OLM → IMAP OST → EML OST → MBOX OST → PST PST → IMAP
See all 30 conversion paths →

Ready to convert your emails?

First conversion free. No credit card required.

Start converting → See pricing