To get MailPlus Server email into Outlook on a local PC, the path is always three steps: export MBOX from MailPlus Server, convert MBOX to PST using Univik Synology MailPlus Converter, then import the PST into Outlook via File then Open and Export then Import/Export. The conversion does not require MailPlus Server to stay online after the export. For large mailboxes (multiple years of email), split the archive by year before converting Outlook PST files above 20 GB slow down noticeably and above 50 GB Outlook may refuse to open them.
This Guide: Local Outlook vs Microsoft 365
This guide covers importing MailPlus Server email into Outlook installed on a local Windows PC for individuals and small teams where each person manages their own email data file.
If you are migrating a company to Microsoft 365 as a cloud service (multiple users, admin panel, Exchange Online), the process is different. That uses Microsoft’s centralised PST Import Service and requires Exchange Admin rights. See our MailPlus to Microsoft 365 migration guide for that workflow.
What You Need Before Starting
Admin access to MailPlus Server on DSM. The MBOX export requires administrator access. If you only have user-level access, you can export your own mailbox as PST via the MailPlus web client but only if your admin has enabled this in the export policy. Ask your admin if you are unsure.
A Windows PC with enough free disk space. The MBOX files, the converted PST files and the working space during conversion all need to fit on the same machine. Rule of thumb: free space should be at least 3x the size of the MBOX export. A 20 GB MBOX archive needs at least 60 GB free to comfortably handle export, conversion and the resulting PST.
Outlook installed on the Windows PC. Outlook 2016 or later handles PST import correctly. Outlook 2013 works but has a smaller recommended PST size ceiling before performance degrades. Microsoft 365 desktop Outlook is the best option.
Step 1: Export MBOX From MailPlus Server
Log into MailPlus Server via DSM and run the admin MBOX export. The export produces a folder containing one subfolder per user account, each with separate MBOX files for Inbox, Sent, Drafts and custom folders.
For the full step-by-step export process including DSM 6 vs DSM 7 differences, see our MailPlus Server export guide. Copy the exported folder to the Windows PC you will use for conversion. The NAS does not need to stay connected after this step.
Step 2: Convert MBOX to PST
Outlook cannot open MBOX files directly. PST is Outlook’s native data file format. The conversion maps the MBOX folder structure to PST folder structure Inbox becomes Inbox, Sent becomes Sent Items and custom folders recreate as named folders inside the PST.
Open Univik Synology MailPlus Converter. Download and install Univik Synology MailPlus Converter on the Windows PC. The software runs entirely offline no NAS connection or internet required after installation.
Load the exported MBOX folder. Click Add Folder and navigate to the top-level folder from the MailPlus Server MBOX export. The converter reads the folder structure and displays all accounts and their email counts. Preview shows each account’s folder hierarchy and message totals before conversion begins.
Select PST as the output format. In the output format dropdown, choose Outlook PST. Configure the output folder a dedicated folder like C:\MailPlus-PST keeps the converted files organised and easy to find.
Apply date filters for large archives. If the account has more than three to four years of email, apply a date filter to convert in yearly or two-year batches. This keeps each PST file at a manageable size. See the PST size section below before deciding whether to filter.
Convert. Click Convert. The converter processes the MBOX files and produces one PST file per account (or per date range if filtered). A conversion log records every file processed. Check the log for any errors before importing into Outlook.
PST File Size and Performance: Split Before You Convert
This is the detail most guides skip. Outlook handles PST files well up to about 20 GB. Above that, performance degrades searches take longer, the application opens more slowly and scrolling through large folders lags. Above 50 GB, Outlook may refuse to open the PST entirely, displaying a “file is not compatible” or “file size limit exceeded” error.
Outlook’s own technical documentation recommends keeping PST files under 20 GB for best performance. Microsoft 365 desktop Outlook has a higher technical limit (50 GB by default, configurable to higher) but still slows noticeably as PST size increases.
The practical rule: if a MailPlus account’s total MBOX files add up to more than 15 to 18 GB, split the PST by year during conversion.
| Total MBOX size for the account | Recommendation |
|---|---|
| Under 10 GB | Convert as one PST. No splitting needed. |
| 10 to 18 GB | One PST is fine but monitor Outlook performance after import. |
| 18 to 35 GB | Split into two PSTs by date range. Import both into Outlook. |
| Over 35 GB | Split into three or more PSTs by year. Consider using the older email as an archive PST that does not load automatically at Outlook startup. |
Archive PST vs primary PST
Outlook lets you have multiple PST files open simultaneously one primary data file and additional PST files added via File then Open and Export then Open Outlook Data File. For large email archives, a common approach is to keep recent email (last 2 years) in the primary PST and import older email into a separate “Archive” PST that sits in Outlook as a secondary folder tree. Users access historical email when needed without it slowing down everyday use.
Step 3: Import PST Into Outlook
Open Outlook. Ensure Outlook is connected to your email account (or can be used offline). The PST import does not require an active email connection it only adds email to the local data file.
Go to File then Open and Export then Import/Export. The Import and Export Wizard opens. Select Import from another program or file and click Next.
Select Outlook Data File (.pst) and click Next. Browse to the PST file produced by Univik Synology MailPlus Converter. For the duplicate handling option, select Do not import duplicates if reimporting a previously imported PST select Replace duplicates only if you are doing a clean first import.
Choose where to import. Select the target mailbox or folder in your Outlook account. Choose Include subfolders to import the full folder structure. Click Finish.
Wait for the import to complete. Outlook shows a progress bar for the import. For large PST files, this takes several minutes to an hour. Do not close Outlook during the import. The import runs in the background you can use Outlook for other tasks while it proceeds, though performance may be slower during the import.
How Folders Map in Outlook After Import
After the PST import, the MailPlus Server folder structure appears in Outlook’s folder pane. Here is what the mapping looks like:
| MailPlus Server Folder | Outlook Folder After Import |
|---|---|
| INBOX | Inbox |
| Sent | Sent Items |
| Drafts | Drafts |
| Trash | Deleted Items |
| Junk / Spam | Junk Email |
| Custom user-created folders | Custom folders with the same name, visible in the folder tree |
| Nested custom folders (subfolder of a custom folder) | Nested subfolder structure preserved |
After import, the PST folders appear as a separate top-level section in the Outlook folder pane (labelled with the PST filename by default). Right-click the PST root and select Data File Properties then Advanced to rename it to something more descriptive like “MailPlus Archive” or the user’s name.
After Import: What to Check
Verify email count in Inbox. Right-click the Inbox folder in the imported PST and select Properties. The folder properties show the total item count. Compare this against the message count for that account shown in the Univik converter before conversion started. A significant discrepancy (more than 1 to 2%) suggests a conversion or import issue.
Open a sample of emails with attachments. Click on five to ten emails that had attachments in MailPlus Server and confirm the attachments open correctly in Outlook. Attachment failures should be zero.
Check that custom folders transferred. If MailPlus Server had custom folders (project folders, client folders, etc.), verify they appear in the Outlook folder tree under the imported PST.
Test search across the imported email. Use Outlook’s search to find a known email by sender or subject. If search returns no results, Outlook may still be indexing the imported PST this can take hours for large archives. Give it time before assuming there is a problem.
Frequently Asked Questions
Can Outlook open MBOX files directly without converting to PST?
No. Outlook does not have a native MBOX import. MBOX is the format used by Unix mail servers, Thunderbird and most Linux email systems. Outlook uses PST (Personal Storage Table) as its native data format. The conversion from MBOX to PST is required there is no shortcut. Univik Synology MailPlus Converter handles this conversion on Windows without any additional tools.
How large can a PST file be before Outlook has problems?
Outlook 2019 and Microsoft 365 desktop Outlook have a default PST size limit of 50 GB. However, noticeable performance degradation starts around 20 GB slower search, longer load times, occasional freezing when scrolling through large folders. Keep individual PST files under 20 GB for smooth everyday use. For archives you access occasionally rather than daily, 20 to 50 GB is acceptable.
What is the difference between importing a PST and opening a PST in Outlook?
Opening a PST (File then Open and Export then Open Outlook Data File) adds the PST as a separate data file visible in the folder pane the email is accessible but stays in the separate PST file on disk. Importing a PST (via Import/Export wizard) copies the email from the PST into an existing Outlook data file, merging it. For MailPlus Server archives you want to keep separate and identifiable, opening is cleaner. For merging into an existing Outlook mailbox, importing is correct.
Does the PST import preserve read/unread status?
Yes. Univik Synology MailPlus Converter preserves read and unread status during the MBOX-to-PST conversion. After import, emails that were unread in MailPlus Server will be unread in Outlook.
Can I import MailPlus Server email into Outlook and then sync it to Microsoft 365?
Yes. If Outlook is connected to a Microsoft 365 account, you can drag and drop email from an imported PST into your M365 mailbox folders in Outlook. The dragged email syncs to M365. This is a manual process suitable for small amounts of email for full organisation migrations see our MailPlus to Microsoft 365 guide.
Conclusion
Importing MailPlus Server email into Outlook takes three steps: export MBOX, convert to PST, import into Outlook. The conversion is the only step that requires any additional software. Once the PST is in Outlook, the email is fully accessible searchable, sortable and browsable exactly as it was in MailPlus Server.
Split large mailboxes before converting. A 30 GB mailbox crammed into one PST file will work technically but make Outlook slow. Two 15 GB PSTs recent email and archive serve the same user better with half the performance impact.
Is this for one user moving their own email or for several people each needing their own import? One user can run through this in an hour. Multiple users on the same machine need to run the process separately for each account’s PST or each person can run it on their own machine using the same MBOX export as the source.