Google Workspace has no MBOX file import. You cannot upload MBOX files to Google Admin and have them land in user mailboxes. Every path from MailPlus Server to Google Workspace runs through IMAP either by connecting Google’s migration tools directly to MailPlus Server over IMAP while it is still running or by converting MBOX to EML using Univik Synology MailPlus Converter and syncing EML into Workspace via an IMAP bridge. Path 1 (direct IMAP migration) is simpler for organisations under 50 users. Path 2 (Google Workspace Migration Tool) handles larger or more complex migrations where MailPlus Server will be decommissioned before migration completes.
The Core Problem: Google Workspace Cannot Import MBOX Directly
Google Workspace’s migration tools connect to email systems over IMAP. They do not accept MBOX files as an upload. This surprises most IT admins planning a MailPlus Server migration they export MBOX files, open the Google Admin console and find no option to upload them.
The reason is architectural. Google’s Data Migration Service, the Workspace Migration for Microsoft Exchange tool (GWMME) and the Google Workspace Migration API all operate as email clients that connect to a source mail server and pull messages via IMAP or Exchange Web Services. They are designed for live server-to-server migration, not for offline file imports.
This means MailPlus Server must either stay accessible over IMAP during the migration or the MBOX files must be converted to a format that can be served over IMAP (using a local IMAP server as a bridge). There is no simpler path for Google Workspace migrations.
If your MailPlus Server is already offline
If the NAS has already been decommissioned and you have MBOX files but no running MailPlus Server, the path requires an IMAP bridge a local IMAP server that serves the MBOX files so Google’s tools can pull from it. See the GWMME section below for how this works. It is more complex than a live migration but it is viable.
Two Paths From MailPlus Server to Google Workspace
Path 1: IMAP Migration
- Google’s migration tools connect directly to MailPlus Server over IMAP
- No MBOX conversion required
- MailPlus Server must stay online and accessible during migration
- Best for under 50 users or where MailPlus Server has a public IMAP endpoint
- Uses Google Data Migration Service in Admin console
Path 2: Google Workspace Migration Tool (GWMME)
- Runs from a Windows machine, connects to MailPlus Server or an IMAP bridge
- Handles larger migrations and staged account batches
- More configuration but better control over migration scope
- Can work after NAS decommission via an IMAP bridge serving the MBOX files
- Better for 50+ users or complex migration requirements
Before You Start: Provisioning and Prerequisites
Both paths require Google Workspace accounts to exist before migration begins. Complete these steps before starting either path.
Provision Google Workspace accounts for all migrating users. In Google Admin, create a user account for every person whose email is being migrated. Assign appropriate Workspace licences. The migration tools map source accounts to these destination accounts by email address or via a mapping CSV.
Confirm MailPlus Server IMAP is accessible. For Path 1, Google’s migration service connects to your MailPlus Server from Google’s infrastructure. IMAP (port 993 with SSL or port 143 with STARTTLS) must be open on the firewall. In MailPlus Server admin, verify IMAP is enabled under Service then MailPlus Client then Protocol.
Export MBOX files as a safety backup before starting. Even when doing a live IMAP migration, export all MailPlus Server mailboxes as MBOX files first. See our export guide for the full process. This gives you a recoverable source if the migration encounters errors or if the IMAP connection is interrupted mid-run.
Decide on the DNS cutover timing. During migration, new email still arrives at MailPlus Server (your MX records still point there). After migration is complete, you update MX records to point to Google Workspace. Decide in advance whether to do a hard cutover (one date) or a phased cutover (some users switch first, others follow). Hard cutover is simpler for smaller organisations.
Path 1 Step-by-Step: IMAP Migration
Open Google Admin and go to Data Migration. Log into admin.google.com with Super Admin credentials. Go to Account then Data migration. Click Set up data migration.
Select Other IMAP server as the source. Choose Email as the migration type. Select Other IMAP server from the source dropdown (MailPlus Server is not listed by name it is a standard IMAP server). Enter the MailPlus Server hostname and choose the correct IMAP protocol and port.
Configure migration scope and date range. Select the date range to migrate all email or email from a specific date forward. For historical archives, migrate everything. For recent email only, set a start date. Google recommends starting migration 30 days before the cutover to keep recent email current.
Add user accounts to migrate. Upload a CSV file mapping source MailPlus usernames and passwords to destination Google Workspace email addresses. Google connects to MailPlus Server as each user to pull their email. Every account needs valid IMAP credentials in the CSV.
Start the migration and monitor progress. Google Admin shows a per-account migration status. Initial migration pulls all email within the scope. After initial migration, Google runs delta syncs to catch new email arriving at MailPlus Server. Run delta syncs until the DNS cutover date.
IMAP migration transfers email only
Google’s IMAP migration transfers email messages and folder structure. It does not migrate contacts or calendar data from MailPlus Server. If users have contacts stored in Synology Contacts and calendar data in Synology Calendar, export those separately as VCF (contacts) and ICS (calendar) files and import into Google Contacts and Google Calendar before the cutover.
Path 2 Step-by-Step: Google Workspace Migration Tool
The Google Workspace Migration for Microsoft Exchange (GWMME) tool runs on a Windows machine and connects to a source server over IMAP or Exchange Web Services. Despite its name, it works with any IMAP-accessible mail source including MailPlus Server.
Download and install GWMME on a Windows machine. Download the Google Workspace Migration for Microsoft Exchange tool from workspace.google.com. Install it on a Windows PC that has network access to the MailPlus Server. The tool does not need to be on the NAS itself.
Configure the source server as an IMAP server. In GWMME, select IMAP as the source type. Enter the MailPlus Server hostname, port and credentials. Test the connection to confirm GWMME can reach MailPlus Server before proceeding.
Authenticate to Google Workspace. Enter your Google Workspace domain and Super Admin credentials. GWMME connects to Workspace via the Google APIs to write email into user mailboxes.
Create account mappings. Upload a CSV mapping source MailPlus accounts to destination Google Workspace accounts. GWMME processes accounts in batches you can migrate in phases if the organisation is large or if some users are ready to migrate before others.
Run the migration and check the migration report. GWMME produces a detailed migration report showing emails migrated, skipped and failed per account. Review the report after migration completes. Skipped emails are typically duplicates or messages exceeding Google’s per-message size limit (25 MB for a single message via migration API).
Which Path to Use
| Situation | Recommended Path |
|---|---|
| Under 50 users, MailPlus Server stays online through migration | Path 1: Google Admin IMAP migration simplest setup |
| 50 or more users or staged migration required | Path 2: GWMME better batch control and reporting |
| NAS already decommissioned, only MBOX files remain | Path 2 with an IMAP bridge serving the MBOX files locally |
| Only personal Gmail, not Google Workspace | See the MailPlus to Gmail guide different tools apply |
After Migration: Verify and Cut Over DNS
Spot check email in Google Workspace before cutting over DNS. Ask two or three users to log into their new Google Workspace account and verify that their email is present and folders look correct. Do not cut over DNS until at least a sample of accounts have been manually verified.
Update MX records to point to Google Workspace. When ready to cut over, update your domain’s MX records in your DNS provider to Google’s mail servers. Google Workspace’s DNS setup guide provides the exact MX record values. DNS propagation typically takes 30 minutes to a few hours depending on your TTL settings.
Run a final delta sync after DNS cutover. Any email that arrived at MailPlus Server after the last migration sync but before DNS propagated will be missed. Run one final delta sync immediately after DNS propagation confirms that new email is arriving at Google Workspace.
Keep MailPlus Server running for 30 days post-cutover. New email routes to Google after MX changes, but users may report missing email during the first few weeks. Keep MailPlus Server accessible so any gaps can be re-migrated from the source.
Frequently Asked Questions
Can I upload MBOX files directly into Google Workspace?
No. Google Workspace has no MBOX file upload. Its migration tools connect to source mail servers over IMAP and pull email directly they do not accept file uploads. If you only have MBOX files and no running MailPlus Server, you need either an IMAP bridge (a local mail server serving the MBOX files) or to convert the MBOX files to EML and use a third-party tool that can import EML into Workspace accounts.
Does Google’s migration tool work with MailPlus Server directly?
Yes. MailPlus Server is a standard IMAP server. Both Google’s Admin Data Migration tool and GWMME work with any IMAP-accessible server. MailPlus Server does not appear in the tool’s source list by name, but selecting “Other IMAP server” and entering the MailPlus Server hostname and credentials works correctly for standard email migration.
Are contacts and calendar data migrated automatically?
No. Google’s email migration tools transfer email only. Contacts stored in Synology Contacts and calendar data in Synology Calendar must be exported separately as VCF and ICS files and imported into Google Contacts and Google Calendar through their own import functions. Handle this before the DNS cutover so users find all their data in Google Workspace from day one.
What happens to email that arrives at MailPlus Server after the migration starts?
Email continues arriving at MailPlus Server until MX records are updated to point to Google Workspace. Google’s migration tools run incremental syncs that pick up new email. Schedule a final sync close to the MX cutover time to minimise the gap. Any email arriving between the last sync and MX propagation can be re-migrated manually if needed.
How long does the migration take for a 50-user organisation?
Migration speed depends on total email volume and IMAP connection speed. A 50-user organisation with moderate email history (50 to 100 GB total) typically completes an initial migration in 12 to 24 hours. Accounts with many years of email or large attachments take longer. Google Admin shows per-account progress watch for any accounts that stall and restart them individually if needed.
Conclusion
Every MailPlus Server to Google Workspace migration runs through IMAP. There is no MBOX file upload path into Workspace. For organisations where MailPlus Server stays online through the migration, the direct IMAP approach using Google Admin’s Data Migration tool is the simplest path. For larger organisations or NAS decommission scenarios, GWMME gives better control and more detailed reporting.
Export MBOX files as a backup before starting either path. If the IMAP migration hits errors, the MBOX files give you a recovery source that does not depend on the MailPlus Server staying online.
Is the MailPlus Server accessible from the internet over IMAP or is it on a private network without a public endpoint? That single constraint determines whether a direct IMAP migration is possible or whether an IMAP bridge is needed.