Three paths move MailPlus Server email to Microsoft 365. Path 1 (PST import): export MBOX from MailPlus Server, convert to PST using Univik Synology MailPlus Converter, upload PST to M365 via the Exchange Admin Center import service. Best for historical email archives. Limit: 10 GB per PST file split large mailboxes before converting. Path 2 (IMAP migration): use Microsoft 365’s built-in IMAP migration wizard to pull email directly from MailPlus Server while it is still running. Best for live migrations of smaller mailboxes. Limit: 500,000 messages total and a 90-day migration window. Path 3 (hybrid): IMAP for recent email, PST import for historical archive. Best for large organisations where IMAP limits would be hit.
Three Paths From MailPlus to Microsoft 365
There is no direct one-click MailPlus-to-M365 migration. Every path involves at least two steps. The right path depends on how much email you have, whether the MailPlus Server will stay online during migration and how much historical email needs to move versus how much can be archived separately.
Before any migration path, you need the MBOX files out of MailPlus Server. See our MailPlus Server export guide for the step-by-step DSM export process. The export is the same regardless of which migration path you choose afterward.
Path 1: MBOX to PST, Then PST Import (Recommended)
This is the most reliable path for complete historical email migration. It works whether the MailPlus Server is still running or already decommissioned making it the right choice for NAS decommission scenarios.
Export MBOX files from MailPlus Server. Log into MailPlus Server via DSM and run the admin MBOX export. See the export guide for the full walkthrough. Copy the exported files to a Windows machine.
Convert MBOX to PST using Univik Synology MailPlus Converter. Open Univik Synology MailPlus Converter, load the exported MBOX folder, select PST as the output format and convert. The converter produces one PST per account with folder structure preserved. The NAS does not need to be connected for this step.
Check PST file sizes before uploading. Microsoft 365’s Import Service enforces a hard 10 GB limit per PST file. Any PST over 10 GB will fail during upload. Check file sizes before proceeding. If a PST exceeds 10 GB, split it by date range re-run the conversion with a date filter to produce separate PSTs for different year ranges. See the PST limits section below for the full process.
Upload PST files to Microsoft 365 via the Import Service. In the Microsoft 365 admin centre, go to Compliance then Data lifecycle management then Import. Use the network upload method upload PST files directly from a Windows machine to the Microsoft Azure storage area via a provided SAS URL using the AzCopy tool. The Microsoft documentation provides the exact AzCopy command. This does not require any additional software licences beyond your M365 subscription.
Create the import job and map PST files to mailboxes. After uploading, return to the Import section in the compliance centre. Create a new import job, download the CSV mapping template and fill in which PST file maps to which M365 mailbox. The mapping file also lets you specify a target folder within the mailbox. Submit the job Microsoft 365 processes imports in the background and emails you when complete.
The 10 GB PST Limit You Need to Know About
Microsoft 365’s Import Service rejects individual PST files larger than 10 GB. This is not a soft recommendation the upload fails if a file exceeds the limit. For users who have run MailPlus Server for several years with active mailboxes, 10 GB per account is easy to exceed.
How to handle it:
Check sizes before converting. Look at the MBOX files for the largest accounts the total size of all MBOX files for one account gives a rough indication of the resulting PST size. Accounts where the MBOX files exceed 8 GB are at risk of producing an oversized PST.
Use date filters to split by year. In Univik Synology MailPlus Converter, apply a date filter to convert each account in two or three date ranges for example, 2019 to 2021 as one PST and 2022 to present as another. This keeps each PST under 10 GB. Update the mapping CSV to point both PSTs to the same M365 mailbox M365 merges the imported email into the mailbox correctly.
Confirm PST file sizes after conversion. Right-click each PST in File Explorer and check the file size in Properties before uploading. Do not assume PST compression can vary. Confirm every PST is under 10 GB before starting the AzCopy upload.
The 10 GB limit applies per PST file, not per mailbox
You can import multiple PST files into the same mailbox. There is no documented limit on how many PST files you map to a single mailbox or on total imported mailbox size (subject to your M365 licence mailbox quota). The restriction is purely on individual PST file size. Splitting one large PST into multiple smaller ones and mapping all of them to the same mailbox is the correct approach.
Path 2: IMAP Migration via Microsoft 365 Admin
Microsoft 365 includes a built-in IMAP migration wizard in the Exchange Admin Center. It connects directly to MailPlus Server over IMAP and pulls email into M365 mailboxes while MailPlus Server is still running. No MBOX export or PST conversion is required for this path.
Confirm MailPlus Server’s IMAP is accessible from the internet. The M365 migration service connects to your MailPlus Server from Microsoft’s infrastructure. IMAP (port 993 for SSL or port 143 for STARTTLS) must be open on your firewall and the MailPlus Server’s IMAP service must be enabled. In MailPlus Server admin, confirm IMAP is on under Service then MailPlus Client then Protocol.
Create M365 mailboxes for each migrating user. In the Microsoft 365 admin centre, provision mailboxes for all users before starting the migration. Assign appropriate licences. The IMAP migration syncs from MailPlus Server into these target mailboxes.
Create the IMAP migration batch in Exchange Admin Center. Go to Exchange Admin Center then Migration then Add then IMAP migration. Provide the MailPlus Server hostname and credentials. Upload a CSV file listing the source usernames, passwords and target M365 mailbox addresses. Start the migration batch.
Run the initial sync, then monitor incremental syncs. The first sync pulls all email. After initial sync completes, M365 runs incremental syncs every 24 hours to catch new email arriving at MailPlus Server. Continue incremental syncs until you are ready to switch MX records and cut over DNS to M365.
Cut over DNS and complete the migration. Once users are ready, update your domain MX records to point to Microsoft 365. New email routes to M365 from that point. Complete the migration batch in Exchange Admin Center. Update Autodiscover settings and reconfigure user mail clients to connect to M365 instead of MailPlus Server.
IMAP Migration Limits and Caveats
The IMAP path has two hard constraints that eliminate it as an option for larger organisations or those with large historical archives.
500,000 message total limit. Microsoft 365’s IMAP migration supports a maximum of 500,000 messages across the entire migration batch. This is approximately 50 active users with four years of normal email volume each. Organisations above this threshold must use Path 1 (PST import) or a third-party migration tool.
90-day migration window. IMAP migration batches run for a maximum of 90 days. If the migration is not completed within 90 days, the batch expires and must be restarted. For large mailboxes or slow connections, 90 days can pass before the initial sync completes on all accounts.
IMAP migrates email only. Contacts, calendar events and tasks in MailPlus Server are not migrated by the IMAP path IMAP is an email protocol. If users have contacts and calendar data in MailPlus Server that needs to move to M365, handle that separately via vCard export and ICS export.
Requires MailPlus Server to stay online. The IMAP path requires MailPlus Server to be accessible throughout the migration. If the NAS is being decommissioned on a specific date, Path 1 (PST import) is more practical it works after the NAS goes offline.
Path 3: Hybrid IMAP for Live Email, PST for Archive
For organisations with more than 500,000 messages but who want the simplicity of IMAP for recent email, a hybrid approach splits the workload:
Use IMAP migration for the last 12 to 24 months of email. Configure the IMAP migration with a start date filter to only pull email from a recent date. This keeps the message count within the 500,000 limit and the initial sync completes quickly. Users get their recent email in M365 immediately.
Use PST import for the historical archive. Export and convert the older email (everything before the IMAP start date) to PST files and import via the compliance centre. This runs in the background while users are already working in M365 and does not block the cutover.
Decide on the archive destination. Historical email from PST import can go into the primary mailbox (mixed with recent email) or into the M365 In-Place Archive (if users have an Exchange Online Plan 2 or M365 E3 licence). The archive keeps the primary mailbox clean for users who are unlikely to need the historical email daily.
Which Path Fits Your Organisation
| Organisation Profile | Recommended Path | Why |
|---|---|---|
| Fewer than 50 users, under 500k messages total | Path 2: IMAP migration | Simplest path, no conversion step, live sync during cutover |
| Any size, NAS being decommissioned on a fixed date | Path 1: PST import | Works after NAS goes offline. No dependency on running MailPlus Server. |
| Over 500k messages or over 10 years of email history | Path 1 or Path 3 (hybrid) | IMAP message limit would be hit. PST handles any volume. |
| Organisation wants users in M365 immediately while archive migrates | Path 3: Hybrid | IMAP gets recent email into M365 fast. PST import handles archive in background. |
| Small team, 1 to 5 users, want email in Outlook on individual PCs | See the MailPlus to Outlook guide | Local Outlook import is a different workflow from M365 tenant migration |
After Migration: Verify Before Decommissioning
Do not decommission the MailPlus Server until the migration is confirmed complete. Run these checks first:
Spot-check email counts per account. In MailPlus Server, check the total message count for three or four representative accounts. In M365, open those same accounts and verify the email counts are within an acceptable tolerance. A small discrepancy (under 1%) is normal completely empty folders in M365 for accounts that had email in MailPlus Server indicates a mapping or import problem.
Check that folder structure transferred. Open a migrated account in Outlook or OWA and confirm that custom folders (not just Inbox, Sent and Drafts) are present. PST import should preserve all folders. IMAP migration preserves folders if they were IMAP-accessible on MailPlus Server.
Verify attachments in a sample of emails. Open five to ten emails with attachments from different accounts and confirm attachments open correctly in M365. Attachment failure rate should be zero if attachments are missing, check the PST conversion log for errors.
Keep MailPlus Server running for 30 days after cutover. Even after MX records point to M365, keep MailPlus Server accessible for 30 days. Users may discover missing email during that window that needs re-exporting. Decommission the NAS only after the 30-day window closes with no reported issues.
Frequently Asked Questions
Does Microsoft 365 have a native import for MailPlus Server MBOX files?
No. Microsoft 365’s Import Service accepts PST files only. MBOX files must be converted to PST before uploading. Univik Synology MailPlus Converter handles this conversion on Windows without requiring MailPlus Server to remain online export the MBOX files first, then convert.
What is the maximum PST file size for Microsoft 365 import?
10 GB per PST file. Any PST larger than 10 GB is rejected during the AzCopy upload phase. Split oversized PSTs by date range before uploading. Multiple PSTs can map to the same target mailbox M365 merges them correctly. There is no documented limit on the number of PSTs per mailbox or on total imported mailbox size.
Can the IMAP migration handle all email including attachments?
Yes. IMAP migration transfers the full email including attachments. The 500,000 message limit applies to the count of messages, not to total data volume. However, very large attachments can cause individual message transfers to time out. If your MailPlus Server accounts contain many emails with very large attachments (50 MB or more), Path 1 (PST import) is more reliable PST conversion handles large attachments without the timeout risk of an IMAP connection.
How long does the PST import take in Microsoft 365?
Typically 1 to 3 days after the import job is submitted, depending on PST size and Microsoft’s current queue. Large organisations importing hundreds of PST files can take longer. Microsoft processes imports in the background you receive an email notification when the import job is complete. Users can continue working in M365 mailboxes while the import runs.
What happens to email that arrives at MailPlus Server after the MX cutover?
After you update MX records to point to Microsoft 365, new email delivers directly to M365. Email that arrived at MailPlus Server before the cutover but was not yet migrated needs to be migrated separately either via a final IMAP sync cycle or by re-exporting and importing the delta as PST. This is why running incremental IMAP syncs up to the cutover date and performing a final sync immediately before the MX change is important.
Conclusion
The right path from MailPlus Server to Microsoft 365 depends on two questions: is the NAS staying online through the migration and how many messages need to move. IMAP migration is the simplest path when the NAS stays online and message counts are under 500,000. PST import is the reliable path for everything else particularly NAS decommission scenarios where the MailPlus Server goes offline on a fixed date.
The 10 GB PST limit is the single most common migration failure point for MailPlus Server to M365 migrations. Check file sizes before uploading and split anything over 8 GB as a precaution.
Is the MailPlus Server staying online through the migration or does it need to be decommissioned before conversion is complete? That single answer points directly to which path is viable.