When a Synology NAS running MailPlus Server is being decommissioned, the email archive must be exported before the hardware goes offline. Export all mailboxes as MBOX files via the MailPlus Server admin interface, copy the files to a Windows machine and the NAS can be powered off. Univik Synology MailPlus Converter converts the MBOX files to PST, PDF or EML offline no NAS connection required. This means the decommission date is independent of the conversion timeline. Export first, power off on schedule, convert at your own pace afterward.
The Key Insight: You Do Not Need the NAS for Conversion
Most IT teams approaching a NAS decommission assume the Synology hardware needs to stay online until every email is fully migrated into its destination system. That assumption creates pressure: the decommission date gets pushed, the NAS sits in the server room past its EOL and the migration becomes a blocker for the hardware refresh.
The correct sequence is different. The NAS only needs to stay online long enough to export the MBOX files. Once those files are copied to a Windows machine, the NAS is done. Conversion from MBOX to PST, PDF or EML happens entirely offline on any Windows machine the process can take days or weeks after the NAS is already powered off and removed. The decommission date and the conversion completion date are independent.
This changes the project timeline substantially. The NAS can go offline on schedule. Email migration completes in the background on a workstation, not on a live server that should have been retired.
What Is at Risk If You Power Off Without Exporting
MailPlus Server stores email in a database on the NAS drives. When the NAS is powered off without exporting:
The email data is still on the drives but it is stranded. The data is not deleted by powering off. However, it is in MailPlus Server’s proprietary database format. Without a running MailPlus Server instance to access it, the data is unreadable by standard tools.
Recovery is difficult and time-consuming. Retrieving email from a powered-off NAS with an unexported MailPlus database requires either reinstalling DSM and MailPlus Server on new hardware to re-access the database or using specialist data recovery services. Both are expensive in time and potentially in cost.
Drive reuse destroys the data permanently. If the NAS drives are wiped and reused before the email is exported, the data is unrecoverable. This is the single most common irreversible mistake in NAS decommission projects.
Do not wipe the drives until conversion is verified
Even after the NAS is powered off and the MBOX files are copied, keep the NAS drives intact until you have verified the conversion output. If a conversion error is discovered after the drives are wiped, there is no recovery path. The verification step described below takes 20 minutes. It is worth doing before the drives are cleared for reuse.
Planning the Decommission Timeline
A decommission project for a NAS running MailPlus Server has four phases. Each phase has a hard dependency on the previous one.
| Phase | Action | NAS Required? | Typical Duration |
|---|---|---|---|
| 1. Audit | List all accounts including deactivated ones. Confirm total mailbox sizes. | Yes | 1 to 2 hours |
| 2. Export | Run MBOX export for all accounts. Copy files to Windows machine. Verify export. | Yes until copy is complete | 2 to 8 hours depending on archive size |
| 3. Decommission | Power off NAS. Remove from server room if needed. | N/A NAS goes offline here | Per your hardware schedule |
| 4. Convert and migrate | Convert MBOX to PST, PDF or EML. Import into destination system. Verify. | No fully offline | Hours to days depending on archive size and destination |
The decommission date (Phase 3) can be set independently of the migration completion date (Phase 4). The only hard dependency is that Phase 2 (export and copy) must complete before Phase 3 (NAS power-off).
Step 1: Audit All Accounts Including Deactivated Ones
Before exporting, get a complete picture of what MailPlus Server holds. Missing a deactivated account at this stage means that email is stranded when the NAS goes offline.
List all active accounts. In MailPlus Server admin, go to User then User List. Export or screenshot the full list of active mailboxes. Note any accounts with unusually large mailbox sizes these will take longer to export and produce larger MBOX files.
List all deactivated accounts. Deactivated accounts former employees, project addresses, role-based accounts that have been disabled are often overlooked in decommission projects. In MailPlus Server, filter the user list to show deactivated accounts. Include all of them in the export. MailPlus Server 3.x and 4.x on DSM 7 explicitly supports exporting deactivated account mailboxes in the same way as active ones.
Check total storage used. In MailPlus Server, go to Storage to see total email storage usage. This figure tells you how much free space you need on the destination Windows machine and gives an estimate of how long the export and copy will take.
Confirm legal hold or compliance requirements. Before decommissioning, check with your legal or compliance team whether any accounts are under a legal hold or subject to retention obligations. Email under legal hold must be preserved in its original form do not delete the NAS MBOX files or the converted output until legal confirms the hold is released. See the governance notes in the FAQ.
Step 2: Export All Mailboxes as MBOX
With the audit complete, run the admin MBOX export from MailPlus Server. This is covered in full detail in our MailPlus Server email export guide including DSM 6 vs DSM 7 interface differences. Key points for the decommission context:
Export all accounts in one run, not sequentially. Select all accounts (active and deactivated) in the MailPlus Server export interface before starting. Running one account at a time is slow and risks missing accounts. A single batch export is faster and produces a complete, organised output folder.
Export to a dedicated shared folder with enough free space. Create a shared folder on the NAS specifically for this export for example, MailPlus-Decommission-Export. Confirm the NAS volume has at least 1.5x the total email storage size in free space before starting. The MBOX export compresses somewhat but attachments can inflate the output.
Copy to Windows machine immediately after export. Do not leave the MBOX files only on the NAS. Connect the Windows machine to the NAS via SMB and copy the entire export folder. Alternatively, copy to an external USB drive connected to the NAS. The copy is complete when the file sizes on the destination match the source verify before disconnecting.
Make a second copy to a different location. Copy the MBOX files to a second location a separate external drive, a NAS on a different device or cloud storage. One copy on a single Windows machine is not a safe backup. Hardware failure during conversion would leave you with nothing. Two copies before the NAS goes offline is the minimum acceptable standard for a permanent decommission.
Step 3: Verify the Export Before Powering Down
This step takes 20 minutes and it is the last safety check before the NAS goes offline permanently. Do not skip it.
Confirm every account has an export folder. Open the export destination on the Windows machine. Count the account folders. Cross-reference against the account list from Step 1. Every account active and deactivated should have a corresponding folder. A missing folder means that account was not exported.
Check that no account folder is empty. Open two or three account folders and confirm MBOX files are present. An empty account folder may indicate the account had no email (acceptable) or that the export process skipped it (investigate before proceeding).
Compare total file size against MailPlus Server storage usage. Add up the sizes of all MBOX files in the export folder. Compare against the storage figure from the MailPlus Server Storage view. The MBOX total will not be identical attachments are stored differently but it should be in the same order of magnitude. A 40 GB MailPlus Server producing 4 GB of MBOX files indicates something went wrong in the export.
Do a quick MBOX readability check. Open Univik Synology MailPlus Converter, load the export folder and let the preview run without converting. The preview shows email counts per account and per folder. Check that the largest account shows a message count consistent with how active that mailbox was. If an account shows zero messages when it should have thousands, investigate before the NAS goes offline.
Once these four checks pass, the NAS can go offline. The email data is safely on the Windows machine (in two copies) and conversion can proceed independently.
Step 4: Convert After the NAS Goes Offline
With the NAS decommissioned, the conversion runs entirely on the Windows machine. Open Univik Synology MailPlus Converter, load the MBOX export folder and select the output format appropriate for the destination:
PST for Microsoft 365 or Outlook
Convert to PST for import into Microsoft 365 via the Exchange Admin Center Import Service or for opening directly in Outlook on a local PC. Check PST file sizes before importing to M365 the 10 GB per file limit applies. Split large accounts by year if needed.
PDF for long-term archive
Convert to PDF for compliance archiving and long-term preservation. PDF is universally readable without any email client. One PDF per email, stored in the original folder structure. Suitable for regulatory retention requirements where email must be preserved but not necessarily accessible in an email client.
EML for platform-agnostic access
Convert to EML for maximum portability. EML files can be opened in Outlook, Thunderbird, Apple Mail and most email clients. Useful when the destination platform is not yet decided or when different users are migrating to different systems.
For the full migration path to Microsoft 365 after conversion, see our MailPlus to Microsoft 365 guide. For import into Outlook on a local PC, see our MailPlus to Outlook guide.
Step 5: Move Email to Its Destination
Where the converted email goes depends on what is replacing the MailPlus Server:
Replacing with Microsoft 365: upload PST files via the M365 compliance centre Import Service. Users access their historical email in Outlook or OWA alongside new email.
Replacing with Google Workspace: the path is MBOX or EML to Gmail via Google Workspace Migration tools. Google’s Data Migration Service does not accept MBOX directly see our Google Workspace migration guide in the cluster for the two-step path.
No replacement archive only: store PST or PDF files on a shared drive or document management system. Users access historical email on request rather than through a live email client. This is appropriate for organisations where the email service has ended entirely and no one needs daily access to the archive.
Replacing with a new NAS running MailPlus Server: see our NAS-to-NAS migration guide. The MBOX export can be imported into the new MailPlus Server instance using the built-in migration tool no conversion step required if staying within the MailPlus Server ecosystem.
Full Pre-Decommission Checklist
List all active MailPlus Server accounts
List all deactivated accounts include them in the export
Confirm no accounts are under legal hold before proceeding
Check available storage on the NAS export destination
Run full MBOX export from MailPlus Server admin all accounts
Copy MBOX export to Windows machine (Copy 1)
Copy MBOX export to a second location (Copy 2 separate drive or cloud)
Verify account folder count matches account list
Verify no account folders are empty (spot check)
Verify total MBOX size is in the expected range vs MailPlus storage
Run converter preview confirm message counts are reasonable
✅ NAS can now go offline
Convert MBOX files to target format (offline, on Windows)
Spot-check conversion output (email counts, attachments, folder structure)
Import into destination system (M365, Outlook, Google Workspace)
Verify import at destination (spot check per account)
Retain original MBOX files for 90 days after verified import before deleting
Once 90-day window passes with no issues reported clear NAS drives for reuse
Frequently Asked Questions
Does the Univik converter require the NAS to be connected during conversion?
No. Once the MBOX files are copied to a Windows machine, Univik Synology MailPlus Converter works entirely offline. No NAS connection, no VPN, no DSM login. The conversion runs locally and produces output in the specified folder on the Windows machine. This is the key advantage for decommission projects the NAS can go offline on its scheduled date regardless of how long the conversion takes.
What happens to email data if the NAS is powered off without exporting?
The email data remains on the NAS drives it is not deleted by a power-off. However, it is stored in MailPlus Server’s internal database format, which is not directly readable without a running MailPlus Server instance. To access it, you would need to either reinstall DSM and MailPlus Server on replacement hardware and restore the database or engage specialist data recovery services. Neither option is quick or guaranteed. Export before power-off is the only reliable approach.
Should we export deactivated accounts as well as active ones?
Yes, always. Deactivated accounts former employees, project mailboxes, role accounts contain email that may have legal, compliance or business value. In decommission projects, deactivated accounts are frequently overlooked, leaving that email stranded when the NAS goes offline. MailPlus Server’s export interface includes deactivated accounts in the account list for exactly this reason. Include all of them in the batch export.
How long does an MBOX export take for a 50-user organisation?
It depends on total email volume and NAS hardware. A 50-user organisation with a modest email footprint (50 to 100 GB total) typically exports in two to four hours. An organisation with heavy email use and large attachments (200 GB or more) can take eight hours or longer. Schedule the export outside business hours and ensure the DSM session does not time out use the MailPlus Server progress view to monitor without keeping the browser active.
Can we split the decommission into phases some accounts now, some later?
Yes. If the NAS needs to stay partially operational (for example, some users are still active while others have already migrated), export and convert the migrated users first and leave the active users until their migration is complete. The MBOX export allows account-level selection. Run it per group rather than all at once if a phased decommission is required. Just ensure the final batch including all deactivated accounts is exported before the NAS goes offline permanently.
Do we need to keep the NAS drives after decommissioning?
Keep the drives physically intact until the 90-day post-import verification window closes with no reported issues. After that point and after confirming that the original MBOX copies are stored in two independent locations, the drives can be securely wiped and reused. Use a certified data erasure tool (such as Univik File Eraser) rather than a simple format a standard format does not prevent data recovery. Secure erasure is the standard for hardware that leaves the organisation.
Conclusion
A NAS decommission with MailPlus Server is a sequenced project, not a simultaneous one. Export and copy come before power-off. Conversion and migration come after. The NAS and the migration timeline are decoupled the moment the MBOX files are on a Windows machine with a second backup copy confirmed.
The checklist above represents the complete sequence. Items 1 through 12 must happen while the NAS is online. Items 13 through 18 happen offline at whatever pace your migration project allows.
When does your NAS need to go offline is there a hard deadline set by a lease, a hardware EOL or a data centre move? The answer to that question determines how much time is available for the export phase and whether a phased account-level approach is needed.