A 5-minute walkthrough of exporting Kerio Connect mailbox data to Outlook PST format using the 4n6 Kerio Server Converter. Useful when retiring a Kerio mail server, moving staff to Microsoft 365 or Google Workspace, or producing forensic-grade archives. Kerio stores everything as standard EML files inside a Store directory tree, which makes the data portable but means a converter is still needed to assemble PSTs.
Video Transcript
Hello and welcome. This video shows how to export Kerio Connect mailbox data to Outlook PST format. Kerio Connect is a commercial email server developed by Kerio Technologies, now owned by GFI Software. Unlike most email servers, Kerio Connect runs on Windows, Linux and Mac OS, and it stores messages as standard EML files inside a Store directory tree. We use the 4n6 Kerio Server Converter for this walkthrough.
The default Kerio storage path on Windows is C:\Program Files\Kerio\MailServer\Store\Mail\domain\username. On Linux it is /opt/kerio/mailserver/store/mail. On Mac it is /usr/local/kerio/mailserver/store/mail.
Each user folder contains subfolders like INBOX, Sent Items, Drafts, plus subdirectories for MyContacts, MyCalendar, MyTasks. Inside each folder is a #msgs subfolder holding the actual EML files, plus index.fld, properties.fld, status.fld for indexing.
Step one. Stop the Kerio Connect server, or copy the Store folder during a quiet maintenance window, so files are not in flux.
Step two. Download and install the 4n6 Kerio Server Converter on a Windows machine. The tool is standalone and does not need Kerio installed locally.
Step three. Launch the tool. Click Choose Folders and browse to the Kerio Store mail directory or to a single user folder.
Step four. The tool reads the EML files from each #msgs subfolder and shows the folder tree on the left. Tick the folders you want to convert. Use the preview pane to inspect individual emails.
Step five. Pick PST as the output format from the Save As dropdown. For multiple users, tick Create Separate PST per User. Other formats include MBOX, EML, MSG, PDF, plus direct upload to Office 365, Gmail, Yahoo, IMAP.
Step six. Set the destination path. Click Convert. The tool processes each folder and produces a PST file ready for Outlook. A Notepad log opens with start time, end time, items converted and any skipped messages. Thanks for watching.
What Kerio Connect Is and Why You Would Migrate Off It
Kerio Connect is a commercial email and groupware server originally built by Kerio Technologies (a Czech software company founded in 1997) and acquired by GFI Software in 2017. It bundles SMTP, IMAP, POP3, ActiveSync, CalDAV, CardDAV plus a custom WebMail interface in one cross-platform package, marketed for years as the "Office Workhouse" alternative to Microsoft Exchange. Unlike most competitors, Kerio Connect runs natively on Windows, Linux and Mac OS, which made it popular with mixed-OS shops.
The most common reasons to export Kerio Connect data to PST or another format: retiring an on-premises Kerio server when moving staff to Microsoft 365 or Google Workspace, archiving departed-employee mailboxes in a portable format, producing forensic-grade evidence for a legal hold, or merging Kerio data with an existing Outlook environment after a company acquisition. In all cases, Kerio's native EML-based storage has to be assembled into something Outlook (PST), Thunderbird (MBOX), or a cloud target (Office 365, Gmail) can ingest.
The tool used in this video is the 4n6 Kerio Server Converter, a Windows desktop application that reads the Kerio Store directory tree directly without requiring Kerio Connect itself to be installed. This matters because most migration projects involve copying user folders from a server to a separate workstation where conversion runs.
How Kerio Connect Stores Mail (Different from MDaemon)
Kerio Connect's storage layout is genuinely different from most email servers, and understanding it helps the migration go smoothly. Each mailbox is a directory tree of standard EML files (RFC 822 format) sitting in the file system. There is no proprietary database. There is no single mailbox file. Each individual email is its own .eml file inside a #msgs subfolder, with separate index files alongside.
This is documented in the official Kerio Connect Administrator's Guide. The choice has practical implications: you can open any single message by double-clicking its .eml file, you can back up the Store folder with any file-system backup tool, but you cannot merge messages into a single Outlook PST without a conversion tool that knows the layout.
Kerio Connect Store Directory Layout (by OS)
| OS | Default Store directory path |
|---|---|
| Windows | C:\Program Files\Kerio\MailServer\Store\Mail\<domain>\<user>\ |
| Linux | /opt/kerio/mailserver/store/mail/<domain>/<user>/ |
| Mac OS | /usr/local/kerio/mailserver/store/mail/<domain>/<user>/ |
| Archive directory (all OS) | <Store>/archive/<period>/ e.g. 2026-Apr, 2026-Q1, depending on archiving interval (daily / weekly / monthly). |
| Inside each user folder | Subfolders for INBOX, Sent Items, Drafts, Trash plus custom user folders. Plus separate trees for MyContacts, MyCalendar, MyTasks, MyNotes. |
| Inside each subfolder | A #msgs/ subdirectory holds the actual .eml files, one per message. Adjacent index files: index.fld (message index), properties.fld (folder properties), status.fld (read-state). |
For a user named raj on the example.com domain on a Windows server, the inbox path is C:\Program Files\Kerio\MailServer\Store\Mail\example.com\raj\INBOX\#msgs\, and that folder contains files like 001.eml, 002.eml and so on. Calendar events for the same user live separately at C:\Program Files\Kerio\MailServer\Store\Mail\example.com\raj\MyCalendar\#msgs\, also as .eml files (Kerio uses EML internally for calendar items too, with iCalendar payloads).
Tool Requirements
| Requirement | Value |
|---|---|
| Tool name | 4n6 Kerio Server Converter |
| Vendor | 4n6 Software (forensiksoft.com) |
| Trial limit | 10 messages per folder, free download |
| Operating system (tool) | Windows 7 / 8 / 10 / 11 (32 or 64-bit), Windows Server 2008 / 2012 / 2016 / 2019. The tool itself is Windows-only, even though Kerio Connect runs cross-platform. |
| Source Kerio Connect versions | Kerio Connect 7.x, 8.x, 9.x, 10.x. Compatible with all GFI Kerio Connect releases. |
| Source Store access | Read access to the Kerio Store/Mail directory tree. The Store folder can be on the same machine, on a network share, or copied to a USB drive. |
| Kerio Connect installation | Not required. Tool reads raw EML files and index files directly. |
| Outlook installation | Not required for PST output. Tool generates PST files standalone. |
| Internet connection | Required only for cloud destinations (Office 365, Gmail, Yahoo, generic IMAP). |
| RAM recommended | 4 GB minimum, 8 GB for large mailboxes (10 GB+ stores). |
| Disk space | 200 MB for the tool, plus output destination space (PST output is roughly the same total size as source EML). |
6 Steps to Export Kerio Connect Mailbox to PST
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Stop Kerio Connect or pick a maintenance window. Reading the live Store directory while Kerio is actively writing produces truncated EML files. On Windows: services.msc, stop "Kerio Connect". On Linux: sudo systemctl stop kerio-connect. On Mac: sudo launchctl unload /Library/LaunchDaemons/com.kerio.kerio-connect.plist. Copy the entire Store folder (or just the user folders you need) to your Windows workstation. Restart Kerio after the copy completes so users keep getting mail.
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Download and install the 4n6 Kerio Server Converter. Visit forensiksoft.com and download the installer. Run it on your Windows workstation. The tool is a single-file installation, takes about 2 minutes. The trial version converts 10 messages per folder so you can verify the workflow before buying a license.
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Load the Kerio Store mail directory. Click Choose Folders and browse to the location where you copied the Kerio Store. To migrate one user, point at Store\Mail\<domain>\<user>\. To migrate every user in a domain, point at the parent Store\Mail\<domain>\ folder. The tool will walk every user subdirectory automatically.
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Preview and select folders. The tool walks each #msgs subfolder, parses every .eml file, and displays the folder tree on the left panel: INBOX, Sent Items, Drafts, custom folders, MyContacts, MyCalendar, MyTasks, MyNotes. Tick the checkboxes next to folders you want to convert. Use the preview pane on the right to inspect individual messages with content view, raw view, hex view or message-header view.
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Pick PST as the output format. From the Save As dropdown, pick PST. For multi-user migrations tick Create Separate PST per User to get one PST per mailbox (the typical pattern for org-wide migration). Optional alternatives: MBOX (Thunderbird), EML (individual files, useful if Outlook is not the destination), MSG (Outlook single messages), PDF (legal preservation), or pick a cloud target: Microsoft 365, Gmail, Yahoo, generic IMAP server. For cloud destinations you will be prompted for an app password if 2FA is enabled.
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Set destination, convert, verify. Set the output folder. Default is Desktop, but for large migrations point at a dedicated drive. Click Convert. The tool processes each #msgs folder and produces PST files. A live progress bar shows item count. When complete, a Notepad log opens listing start time, end time, items converted, destination path and any skipped items. Open the resulting PSTs in Outlook to verify folder structure and message counts match the source.
Manual EML Export vs Tool Conversion
Because Kerio uses standard EML files, manual export is technically possible. People often try it first then come back to a tool. Here is what the manual approach looks like and why it usually does not finish.
| Approach | Pros and cons |
|---|---|
| Manual: drag and drop EML files into Thunderbird | Pros: free, no tool needed. Cons: Thunderbird does not natively bulk-import a folder of EML files into Local Folders. You need the ImportExportTools NG add-on. Even then, calendar (MyCalendar) and contacts (MyContacts) need different handling. Works for one user with under 1,000 messages. Painful past that. |
| Manual: Outlook drag and drop | Pros: works for tiny mailboxes. Cons: Outlook does not bulk-import EML files at all by default. You have to drag them one by one, or use a PowerShell script with Outlook COM automation that takes hours per mailbox. Folder hierarchy is lost. Calendar items become orphan emails instead of calendar entries. |
| Manual: WebMail "Export to EML" + Outlook drag | Pros: works without server access. Cons: each WebMail user has to do their own export. Output is per-folder EML batches. No bulk option. Cumbersome for a 50-user organisation. |
| Manual: IMAP migration with Outlook | Pros: works while Kerio is live, no Store access needed. Cons: very slow over network, may take days for large mailboxes, IMAP often loses some flags and labels. Requires both source IMAP and destination IMAP / Outlook account. Best when you cannot get to the Store folder. |
| Tool conversion (this video) | Pros: walks the entire Store tree in one pass, preserves folder hierarchy, handles MyContacts and MyCalendar correctly, generates PST per user, supports cloud upload. Cons: paid tool ($99 personal). Trial limited to 10 messages per folder. Windows-only. |
Common Errors and Fixes
| Error or symptom | Cause and fix |
|---|---|
| "Folder not recognised as Kerio Store" | You pointed the tool at the wrong folder. The correct folder is the user mailbox folder (containing INBOX, Sent Items subfolders), not the parent Kerio installation folder. Browse to Store\Mail\<domain>\<user>\. |
| Some EML files truncated or corrupted | The Store folder was copied while Kerio Connect was still running and writing. Stop Kerio Connect first, then copy. If you cannot stop the live server, use Kerio's built-in archiving feature (Configuration > Archiving and Backup) to produce a clean archive copy and convert from that. |
| Calendar items appear as plain emails in Outlook PST | The MyCalendar folder was converted as if it were a mail folder. The tool needs to recognise it as a calendar source. In the converter UI, expand the user tree and verify MyCalendar is shown with a calendar icon, not a folder icon. If still wrong, convert MyCalendar separately and pick "Calendar (ICS)" as the output format, then import the ICS file into Outlook calendar. |
| "Cannot create PST. Outlook installation problem" | The tool generates PST files standalone and does not need Outlook. If this error appears on a 64-bit Windows where 32-bit Outlook is also installed, try the 32-bit installer of the converter. Alternative: output to MBOX first and convert MBOX to PST in a separate step using 4n6 MBOX to PST Converter. |
| Index.fld and #msgs counts do not match | The Kerio index is out of sync with the actual files. This happens when the Store folder is restored from a partial backup, or when manual file changes were made. Ignore the index and let the converter walk #msgs directly. The tool reads the raw EML files, not the index. |
| "Access denied" reading Store folder | The Store folder has restricted ACLs from the original Kerio service account. On Windows, take ownership of the copied folder: right-click > Properties > Security > Advanced > Change Owner. Or run the converter as Administrator. |
| Cloud upload (Office 365) fails with auth error | You used your regular account password instead of an app password. For Office 365 with 2FA enabled, generate an app password and paste that into the tool's credentials field. See our Outlook app password guide. |
| Conversion runs out of disk space | PST output is roughly the same size as the source EML total. For a 100 GB Store, allocate at least 110 GB at the destination. Pick "Create Separate PST per User" to produce many smaller PSTs that can be moved off as they finish. |
| Custom folder names with special characters disappear | Some Kerio folder names use slashes, colons or non-ASCII characters that PST cannot accept. The tool sanitises these by replacing offending characters with underscores. Check the conversion log for "renamed" entries and verify the renamed folders match what you expect inside Outlook. |
Tool Comparison: 4n6 vs Other Kerio Converters
Several vendors make Kerio converter tools. Most are similar in scope (input Kerio Store, output PST and other formats) but differ in price, supported destinations and forensic features.
| Tool | Notes |
|---|---|
| 4n6 Kerio Server Converter (this video) | $99 personal / $299 business / $499 enterprise. 30+ output formats. Forensic preview (hex, raw, header view). Trial: 10 messages per folder. Companion cloud-migration tools available: 4n6 Kerio to Office 365 Migration and 4n6 Kerio to Gmail Converter. |
| RecoveryTools Kerio Migrator | Similar pricing range. Strong cloud migration support (Office 365, Gmail, Workspace). Trial: 25 messages per folder. Windows only. Useful for organisations doing cloud-first migrations. |
| BitRecover Kerio Converter | ~$89 starter. 25+ saving formats. 30-day money-back guarantee. Trial: 25 messages per folder. Strong format coverage including ZOOK-style EML to PST workflow. |
| Transend Migrator (Kerio to Office 365) | Enterprise-focused, scriptable, supports the lettered-drive mapping pattern documented in Transend's Kerio to Office 365 reference guide. Higher price. Used by IT consultancies for large enterprise migrations with custom variable substitution. |
| Shoviv IMAP Backup and Restore | Different category: IMAP-based instead of Store-folder-based. Pulls Kerio data over IMAP while the server is live, so no need to stop Kerio. Useful when you do not have file system access to the Store folder. Slower for large mailboxes. |
| SysTools (no dedicated Kerio product) | SysTools does not ship a dedicated Kerio Connect converter. Their Kerio coverage works via the SysTools IMAP Migration tool (used while the source server is still running and IMAP is enabled). Picked when the Kerio server is live and IMAP-accessible; not suitable when only Store folder backups remain after server decommissioning. |
Real Migration Scenarios
| Scenario | Approach and notes |
|---|---|
| SMB retiring on-premises Kerio Connect, moving to Microsoft 365 | The most common case. ~20-100 user mailboxes, total Store size 30-300 GB. Output: Office 365. Recommended approach: stop Kerio over a weekend, copy the Store folder to a Windows workstation with a fast SSD, run the 4n6 converter with Create Separate PST per User to a local drive first, then upload PSTs to Office 365 via Microsoft's Network Upload service or AzCopy. 4n6 also offers a direct cloud-migration product, the 4n6 Kerio to Office 365 Migration, if you prefer Kerio-mailbox-to-cloud directly without the PST intermediate step. The PST-intermediate approach is faster on slow Indian residential broadband. |
| SMB retiring Kerio, moving to Google Workspace | Output: PST or MBOX intermediate, then upload via Google Workspace Data Migration Service. Or use the 4n6 Kerio to Gmail Converter for direct Gmail-mailbox-to-Workspace transfer. The MBOX intermediate path tends to be more reliable for very large stores because Google's DMS handles MBOX uploads natively. |
| Cross-platform migration: Linux Kerio to Windows Outlook | Common in mixed-OS Indian SMBs. The Kerio Store lives at /opt/kerio/mailserver/store/mail/ on Linux. Copy the entire store/mail directory to a Windows workstation via scp, rsync, USB drive, or a temporary Samba share. EML is portable across operating systems so no path translation is needed. Run the 4n6 converter against the copied folder on Windows. |
| Mac Kerio Connect to Outlook 365 | Uncommon but happens with creative agencies running Mac Kerio. Store at /usr/local/kerio/mailserver/store/mail/. Copy via SMB share or external drive to a Windows VM. Conversion runs on Windows side. Output PST imports cleanly to Mac Outlook 2024 if needed. |
| Legal e-discovery / forensic preservation | One or two specific Kerio mailboxes need preservation for litigation. Output: PDF or EML+MSG combined. The 4n6 forensic preview features (hex view, raw view, header view) are useful here. Each preserved email becomes a court-admissible PDF with full RFC 822 headers. The companion 4n6 Email Forensics tool provides deeper analysis on the converted output. Keep the original Store folder copy under chain-of-custody with sha256 hashes recorded at copy time. |
| Departed-employee archive | Single mailbox, 5-50 GB. Output: PST stored on encrypted external drive. Common pattern: when an employee leaves, IT freezes their Kerio mailbox, copies just that user folder out of the Store, runs conversion to PST, archives per HR retention policy, then deletes the live Kerio account. |
| Server consolidation: Kerio to on-premises Exchange | Less common but happens when SMBs consolidate to Exchange Server. Output: PST intermediate with bulk PST import via Exchange Mailbox Import Request PowerShell, or live IMAP migration via tools like SysTools IMAP Migration while both servers are running. |
Pre-Migration Checklist
| Check | Why it matters |
|---|---|
| Total Store folder size on disk | Determines destination drive requirement. PST output is roughly the same total size as source EML. For 100 GB Store, allocate 120 GB free space. |
| Largest individual mailbox | If any single user mailbox exceeds 20 GB, run that one separately. Very large mailboxes are more reliable as their own conversion job. |
| Active vs stopped Kerio status | If Kerio Connect is still serving live mail, schedule a maintenance window to stop it during the Store copy. Reading a live Store leads to truncated EML files. |
| OS of source server | If Kerio runs on Linux or Mac, plan a copy step to a Windows workstation. The 4n6 converter is Windows-only. |
| Shared folders / public folders | Kerio Connect supports shared mailboxes and public folders. These live under separate Public trees in the Store. Decide in advance whether to migrate them to Outlook public folders, M365 shared mailboxes, or skip them entirely. |
| Calendar and contacts handling | MyCalendar and MyContacts are technically EML-format but with iCalendar / vCard payloads. The 4n6 converter handles them natively but verify in the preview before bulk conversion. |
| Antivirus exclusion | Add the destination folder to your antivirus exclusion list during conversion. Real-time scanning of every newly written PST file slows conversion significantly. |
| Backup of original Store folder | Always copy the Kerio Store folder to a separate location before running the converter, so you preserve the original in case anything goes wrong. |
| Trial run with one user first | Convert one small mailbox end to end before kicking off the full batch. Reveals issues like permissions, app passwords, custom folder names. |
| Test the output | Open the resulting PST in Outlook (or MBOX in Thunderbird) and check that messages render correctly, attachments are accessible, calendar items appear in calendar (not inbox), and folder structure is preserved. |
đź’ˇ Pro tips for Kerio Connect migrations
- Always work from a copy of the Store folder, never directly against the live server. EML files are individual files so a single bad copy step can corrupt only one message, but it is still safer to work on a full duplicate.
- Stop Kerio during the Store copy if at all possible. Even a 5-minute maintenance window during off-hours guarantees you get a clean copy. Restart Kerio after the copy. Conversion itself can run for hours afterwards while Kerio serves users normally.
- Use rsync (Linux/Mac) or robocopy (Windows) for the Store copy step. Both verify file integrity and resume on failure. Plain cp or Explorer drag-and-drop can silently skip files on permission errors.
- Pick "Create Separate PST per User" for org-wide migrations. One PST per mailbox is much easier to import into Office 365 or hand to individual users than one giant merged PST.
- For Office 365 cloud destinations, use Microsoft's Network Upload tool for the final PST-to-cloud step. It is faster and more reliable than direct cloud upload from the converter, especially over slow Indian residential or shared office broadband.
- Calendar items render correctly only if MyCalendar is treated as a calendar source. If items appear in the inbox of the resulting PST instead of the calendar, re-run conversion with MyCalendar selected explicitly and output set to ICS first, then import that ICS into Outlook calendar manually.
- Preserve the Kerio Store folder for at least 90 days after conversion. If a user reports missing emails later, you can re-extract from the original.
- If you also need to migrate other email systems, see our companion guides on converting MDaemon emails, backing up Gmail / Yahoo / Outlook in one tool and converting Thunderbird to Outlook.
- Document the destination folder structure before conversion. Kerio users sometimes have deep custom folder hierarchies. Verify those land in the right place in Outlook after conversion.
- For forensic preservation always pick PDF as the output format and keep the original Store folder under chain-of-custody. Compute sha256 hashes of the original Store directory before any copy, so you can prove the files were not modified.
Frequently Asked Questions
Where does Kerio Connect store mailbox data?
Kerio Connect stores each mailbox as a directory tree of EML files. Default storage paths by OS: Windows: C:\Program Files\Kerio\MailServer\Store\Mail\<domain>\<user>\. Linux: /opt/kerio/mailserver/store/mail/<domain>/<user>/. Mac: /usr/local/kerio/mailserver/store/mail/<domain>/<user>/. Each user folder contains subfolders like INBOX, Sent Items, Drafts, plus the #msgs sub-subfolder where the actual .eml files live. Index files (index.fld, properties.fld, status.fld) live alongside #msgs.
Why does Kerio store messages as EML files?
EML is the RFC 822 standard email message format. Kerio Connect uses it because EML is portable, human-readable, easy to back up at the file system level, and trivially recoverable if the database index breaks. Most other email servers use proprietary formats or single-file databases (Exchange uses EDB, MDaemon uses MSG inside a custom store). Kerio's choice means you can open any individual message in any standard email client by double-clicking the .eml file, which simplifies forensic and recovery work.
Can I just copy EML files manually instead of using a converter?
You can open individual EML files manually with Outlook, Thunderbird or any RFC 822-compliant client by double-clicking. But for a real migration this approach has serious limits: Outlook does not let you bulk-import a folder of EML files into a PST without scripting, calendar (MyCalendar) and contacts (MyContacts) are stored in separate non-mail directories that need different handling, the folder hierarchy from Kerio (INBOX, Sent Items, custom folders) needs to be reconstructed inside Outlook manually, and large mailboxes with 10,000+ EML files take hours of drag-and-drop work. A converter tool automates all of this in one pass.
Do I need Kerio Connect installed to run the converter?
No. The 4n6 Kerio Server Converter is a standalone Windows application. It reads the Kerio Store directory contents directly without needing the Kerio Connect server software installed. This matters because most migration projects involve copying user folders from a decommissioned or offline Kerio server to a Windows workstation where conversion is run separately. You only need read access to the Store\Mail directory tree.
What output formats does the converter support?
Email formats: PST (Outlook), MBOX (Thunderbird, Apple Mail), EML (individual messages, Mailbird, eM Client), MSG (single Outlook messages), EMLX (Apple Mail). Document formats: PDF, HTML, MHT, RTF, DOC, DOCX, TXT, CSV, vCard, ICS. Cloud destinations: Microsoft 365, Office 365, Gmail, Google Workspace, Yahoo Mail, Outlook.com, Exchange Server, generic IMAP server. The 4n6 Kerio Server Converter supports 30+ destinations in total.
Should I convert before or after stopping Kerio Connect?
Always copy the Store folder while Kerio Connect is stopped, or during a maintenance window when no new mail is being delivered. Reading the live Store directory while Kerio is actively writing to it can produce truncated EML files and inconsistent index states. Once you have a clean copy of the Store folder on a separate workstation, restart the Kerio Connect server so users keep getting mail, then run the converter against your copy. The conversion itself does not need Kerio to be stopped.