Migrating IMAP Email Servers to Office 365 Exchange Online

IMAP to Office 365 Migration: Step-by-Step Guide (2026)

IMAP migration moves email from Zimbra, Dovecot, hMailServer, or any IMAP host to Microsoft 365 Exchange Online. Microsoft's native IMAP migration tool migrates email only — not contacts or calendars. A full migration takes 1–4 weeks depending on mailbox count and data volume. This guide covers all five steps end-to-end.

Key facts

• IMAP migration moves email only. Contacts and calendar events require manual export or a third-party tool.
• Microsoft 365's built-in IMAP migration batch tool supports batches of up to 2,000 mailboxes at once.
• You must create Microsoft 365 user accounts and provision licenses before running the IMAP migration.
• DNS cutover (MX record change) is the final step — complete it only after email is fully migrated.
• EPC Group: 29 years of Microsoft consulting, 10,000+ successful deployments including Exchange and M365 migrations, four Microsoft Press books on Microsoft technologies.

Pre-migration checklist

Complete these steps before touching any mailbox data.

• Verify your IMAP server is reachable from Microsoft 365 (allow Microsoft's IP ranges through your firewall).
• Create all Microsoft 365 user accounts. Assign Exchange Online licenses.
• Verify your domain in Microsoft 365 Admin Center → Settings → Domains.
• Document every mailbox: display name, email address, IMAP username, and current mailbox size.
• Communicate the migration timeline to users. Confirm they know the cutover date.
• Plan for an email coexistence period: during migration, mail continues to flow to the IMAP server. After MX cutover, it flows to Exchange Online.

Step 1 — Create IMAP migration endpoint

1. In the Exchange Admin Center (EAC), go to Migration → Endpoints → New.
2. Select IMAP as the endpoint type.
3. Enter the IMAP server FQDN (e.g., imap.yourcompany.com) and port (typically 993 for SSL, 143 for TLS).
4. Set authentication type: Basic, NTLM, or GSSAPI depending on your IMAP server.
5. Test the endpoint connection and confirm it shows Connected status.

Step 2 — Create a migration batch

1. Prepare a CSV file with three columns: EmailAddress, UserName, Password (IMAP credentials for each mailbox).
2. In EAC → Migration, click New → Migrate to Exchange Online → IMAP migration.
3. Upload the CSV file. Select the IMAP endpoint you created in Step 1.
4. Name the migration batch (e.g., "IMAP-Batch-001") and set the target folder (Inbox).
5. Choose to start the batch automatically or manually. For large migrations, start manually after verifying the first 10–20 mailboxes complete successfully.

Step 3 — Monitor migration progress

IMAP migration runs incrementally. It syncs email from the IMAP server to Exchange Online until you complete the batch.

• Check migration status in EAC → Migration → select your batch → View details.
• Monitor for errors: incorrect IMAP credentials, mailboxes exceeding size limits, connection timeouts.
• Re-run failed mailboxes by editing the CSV to include only the failed accounts and creating a new sub-batch.
• Migration speed: Microsoft's IMAP migration tool processes approximately 10 GB per mailbox per hour under normal conditions.

Step 4 — Configure DNS for MX cutover

Do this step only after all mailboxes have been migrated and verified.

1. In your DNS provider, change the MX record for your domain to point to yourtenantname.mail.protection.outlook.com.
2. Set the MX TTL to 3,600 seconds (1 hour) before cutover day to minimize propagation time.
3. After cutover, new email routes to Exchange Online. The IMAP server no longer receives mail.
4. Wait 24–48 hours for DNS propagation to complete globally before decommissioning the IMAP server.

Step 5 — Complete and clean up

1. In EAC → Migration, select your batch and click Complete. This stops the incremental sync.
2. Verify that all mailboxes show Synced status and no items remain pending.
3. Remove the IMAP migration endpoint if no further migrations are planned.
4. Update Autodiscover DNS records so Outlook clients reconfigure to Exchange Online automatically.
5. Decommission the IMAP server after a 30-day retention period (in case users report missing items).

Common IMAP migration problems and fixes

• Connection refused — check firewall rules on the IMAP server. Add Microsoft 365 IP ranges to the allow list.
• Authentication failures — verify IMAP credentials in the CSV. Some IMAP servers require app-specific passwords if 2FA is enabled.
• Items missing after migration — IMAP migration copies items from the Inbox and subfolders but may skip items in non-standard folders. Verify folder structure on the source server.
• Duplicate emails — IMAP migration can create duplicates if users have both old and new clients open during migration. Advise users to use only one client until cutover completes.
• Large mailboxes timing out — for mailboxes over 50 GB, split into multiple migration batches. Migrate in off-hours to avoid throttling.

Frequently asked questions

Does IMAP migration move contacts and calendar events?

No. IMAP migration moves email only. For contacts and calendar events, export them from the source system (PST, vCard, iCal) and import into the user's Outlook or Exchange Online mailbox manually or via a third-party migration tool like BitTitan MigrationWiz.

How long does an IMAP migration take?

A 100-mailbox organization with average mailbox sizes of 5–10 GB takes 3–5 days for initial sync. A 1,000-mailbox migration takes 2–4 weeks. Plan for an incremental sync period (1–2 weeks) where new emails continue to sync before DNS cutover.

Can I migrate shared mailboxes via IMAP?

Yes, if the shared mailbox has its own IMAP credentials. Include the shared mailbox's IMAP username and password in the CSV. Create the shared mailbox in Exchange Online before the migration batch runs.

What is the alternative to the built-in IMAP migration tool?

For large or complex migrations, EPC Group uses purpose-built migration tools that handle email, contacts, calendars, and public folders in a single pass. These tools provide better error reporting, conflict resolution, and status dashboards than the native EAC migration wizard.

Do users lose email during the MX cutover?

No. There is typically a brief period (minutes to hours) during DNS propagation where some email may go to the old server. The old IMAP server should remain active for 48 hours after MX cutover. Any email delivered to the old server during this window can be forwarded or manually moved.

Get expert help with your IMAP migration

EPC Group has completed thousands of email migrations including IMAP, Google Workspace, and on-premises Exchange to Microsoft 365. Call (888) 381-9725 or request a 30-minute discovery call.