IMAP Migration to Office 365: The Complete Step-by-Step Guide

• Last updated: 02 November 2026
• Migration
• Editorial Team

IMAP migration is the simplest way to move email from almost any mail server into Microsoft 365, and it is also easy to get subtly wrong. Microsoft builds the tool into the Exchange admin center, it copies messages from an IMAP server into Exchange Online mailboxes you have already created, and it keeps syncing once a day until you delete the batch. What it doesn’t do matters just as much: it moves email only, with hard limits on item counts and message size, and it leaves contacts, calendars, and tasks behind.

This guide walks through the whole process, from the exact CSV format and migration endpoint to batches, the 72-hour rule, Gmail’s app password requirement, and the mistakes that cause failed cutovers. Every technical detail was checked against Microsoft’s current IMAP migration documentation, and where older guides said something different, we have corrected it. If you are planning more than email, our broader migration checklist covers the rest of the project.

Why Move from IMAP to Office 365?

Organizations move from plain IMAP hosting to Microsoft 365 to get a full workspace instead of a mail-only service: Exchange Online mailboxes with calendars and contacts, plus Teams, OneDrive, SharePoint, and Microsoft’s security and compliance tools. A basic IMAP mailbox stores and syncs messages. It is not designed to share calendars, host meetings, store files, or enforce retention.

• One platform. Email, calendar, contacts, chat, meetings, and file storage live in one subscription instead of several tools.
• Compliance and retention. Microsoft Purview provides retention, eDiscovery, and data loss prevention, which many basic IMAP hosts don’t offer.
• Security controls. Multi-factor authentication, threat protection, and device management are available across the tenant.
• Less to run. No mail server to patch, back up, or keep online, and mobile access is built in.

What Is IMAP Migration to Office 365, and How Does the Data Move?

IMAP migration is Microsoft’s built-in method for copying email messages from any IMAP-enabled mail server into existing Exchange Online mailboxes, using a migration batch you run from the Exchange admin center. It is the standard route when your source system supports IMAP but offers no better migration path.

Behind the scenes, Exchange Online connects to your source server through a migration endpoint, signs in to each mailbox using the credentials in your CSV file, and copies the mail across in an initial sync. After that, it synchronizes each mailbox once a day until you delete the batch. If you migrate with admin credentials, or you know each user’s password, there is no impact on users until you shut down the source system.

One rule comes before everything else: the target mailboxes must exist first. Each user needs an account and a license that includes an Exchange Online plan, and if you plan to use your custom email domain in Microsoft 365, you need to add it as an accepted domain before you create the users. Our walkthrough of adding your domain first covers that step.

Is IMAP Migration the Right Method for Your Source?

IMAP migration fits any standard IMAP server and consumer Gmail, but other sources have better paths that move more than email. Choosing the right method up front avoids redoing the work later.

Your Source	Best Method	Why
Standard IMAP host (Dovecot, Zimbra, hosted webmail)	IMAP migration	Works with any IMAP server, email only
Consumer Gmail	IMAP migration with app passwords	Microsoft’s documented route for consumer Gmail
Google Workspace organization	Dedicated Google Workspace migration	Also moves calendar and contacts
On-premises Exchange	Cutover, staged, or hybrid migration	Moves full mailboxes, not only email
Exported PST files	PST import	No live server to connect to

If you are leaving Google Workspace, Microsoft’s own guidance points organizations to a dedicated migration, and our full Google Workspace migration guide covers it. If your source is a self-managed Exchange server, compare the hybrid, cutover, and staged options before settling on IMAP.

What Does IMAP Migration Move, and What Doesn’t It?

It moves email from the inbox and other mail folders only, up to 500,000 items per mailbox from newest to oldest, with messages up to 35 MB, and it does not move contacts, calendar items, or tasks. These limits come straight from Microsoft’s documentation, and they decide whether IMAP is enough for your project.

• Email only. Contacts, calendar items, and tasks are not migrated, so plan another route for them.
• 500,000 item cap. Mail is copied from newest to oldest, so in a mailbox beyond the cap, the oldest messages are the ones left behind.
• 35 MB message limit. The largest message you can migrate is 35 MB.
• Slash folders. Folders with a forward slash in the name are not migrated until they are renamed.
• Policy side effects. The tool doesn’t recognize retention or archive policies on the source, so messages those policies delete or move can be flagged as missing. That looks like data loss when it isn’t, which makes real loss harder to spot. Microsoft strongly recommends disabling all messaging records management and archival policies before you migrate.

What Do You Need Before You Start?

Before the first mailbox syncs, you need licensed target mailboxes, the source server’s name and credentials, a source server that can handle the connections, and a plan for the policies, DNS, and users the migration touches. Most failed migrations skipped something on this list.

• Create and license every user first. Each needs an Exchange Online plan, and you need to be a global admin to complete the migration steps.
• Confirm IMAP is enabled on the source, and note the server name, such as imap.contoso.com.
• Choose your credentials approach. Admin credentials are least disruptive; the next section explains the formats.
• Raise connection limits. Many servers and firewalls cap total connections, connections per user, and connections per IP address. If you migrate with an admin account, every connection comes from that one account, so its limit matters most. Exchange 2010 and 2007 servers ship with low defaults.
• Disable retention and archive policies on the source, as covered above.
• Lower your MX record’s time to live to 3,600 seconds (one hour) or less before you start, so the final switch propagates faster.
• Brief your users. Tell them messages over 35 MB won’t migrate, ask them to clear old mail and back up their inboxes, and tell them which folders won’t come across.
• Run a test batch. Migrate a few mailboxes first to verify the CSV format, the endpoint, your credentials, the number of simultaneous connections your server can take, and how long a batch takes.

Should You Migrate with Admin Credentials or User Passwords?

Admin credentials are the safest option because users never have to hand over or reset passwords, but they work only on source servers that support them, and each uses a different username format. In the CSV, the UserName column combines the user’s name with the admin account.

• Microsoft Exchange (IMAP): Domain/Admin_UserName/User_UserName
• Dovecot and other SASL servers: User_UserName*Admin_UserName
• Mirapoint: #user@domain#Admin_UserName#
• Courier IMAP and Oracle IMAP: no admin credential support. Set up virtual shared folders and add the optional UserRoot attribute that points to each user’s mailbox location.

If your server doesn’t support admin credentials, you need each user’s password, and if you don’t know them you must reset them to ones you do. Users who don’t learn the new password lose access to their old mailboxes unless you share it after migration. There is also a trap. If a user changes their password before their mailbox migrates, the migration fails. If they change it after, new mail arriving on the old server won’t be copied. Consider changing passwords globally and blocking further changes until migration finishes.

How Do You Migrate IMAP Email to Office 365, Step by Step?

You build a CSV, create an IMAP migration endpoint, then create and start a migration batch in the Exchange admin center, and finish by switching your MX record and deleting the batch after at least 72 hours.

• Find the IMAP server name. Get the full name of the source server, such as imap.contoso.com. Your host’s documentation has it, and if your source offers Outlook on the web, it lists the server under Settings, then POP and IMAP.
• Build the CSV file. Create a file with three headings exactly as written, with no spaces: EmailAddress, UserName, and Password. EmailAddress is the Microsoft 365 mailbox, UserName is the sign-in name on the source system, and Password is the source mailbox password. Add one row per mailbox, up to 50,000 rows and 10 MB, and save it as a CSV.
• Create the migration endpoint. In the Exchange admin center, go to Migration, select Endpoints, and add a new endpoint. Choose IMAP, give the endpoint a unique name, enter the IMAP server name, and leave the remaining settings at their defaults, which work for most sources.
• Create and start the migration batch. In Migration, choose Add migration batch, name it with no spaces or special characters, and select Migrate to Exchange Online with IMAP migration as the type. Confirm the prerequisites, pick your endpoint, import the CSV, set any filtering options, choose your reporting and start options, then save.
• Monitor the batch. Select the batch and choose View details to see each mailbox’s status. If a mailbox fails, check its status report and confirm the EmailAddress column holds the correct Microsoft 365 address.
• Verify, switch mail flow, and finish. Ask users to sign in with their temporary password, set a new password and their time zone, send a test message, and confirm their folders. Then point your MX record at Microsoft 365, wait at least 72 hours, and delete the batch to stop syncing.

Two alternatives are worth knowing. For fewer than 50 mailboxes, the IMAP migration experience in the Microsoft 365 admin center can replace the CSV file. For repeatable or large runs, the migration batch cmdlets in Exchange Online PowerShell give you scriptable control. Our explainer on how migration endpoints work goes deeper on step 3.

How Does the Daily Sync Work, and When Should You Delete the Batch?

After the initial copy, Exchange Online synchronizes each mailbox with its source once a day until you delete the batch, which is why Microsoft says to let a batch run at least 72 hours before deleting it. This is the part of IMAP migration people most often misunderstand.

• It is not a one-shot copy. The daily sync keeps copying mail that lands on the source server, which covers you while MX changes spread.
• MX changes take time. It can take up to 72 hours for other organizations’ mail systems to recognize your new MX record.
• Deleting stops everything. After you delete the batch, mail sent to the old mailboxes is not copied to Microsoft 365, so delete only when every user is working exclusively in Microsoft 365.
• Leave the mailboxes alone. Don’t delete migrated mailboxes or change their SMTP addresses until the batch is deleted, or the migration reports errors.

How Do You Migrate Gmail to Office 365 with IMAP?

Consumer Gmail migrates with IMAP and an app password for each account, while Google Workspace organizations should use Microsoft’s dedicated Google Workspace migration, which also moves calendar and contacts. Microsoft’s IMAP instructions for Gmail are written for consumer accounts.

• Use app passwords. Google treats Outlook as a less secure app and won’t accept a password alone, so each account owner must create an app password, which requires 2-step verification. Workspace admins need to let users turn it on. Older guides that tell you to switch on “less secure apps” are out of date.
• Put the app password in the CSV. The Password column holds the app password, not the account password.
• Expect Google’s limits. If a large mailbox shows Syncing for a long time, you may be hitting Google’s bandwidth limits. Unlocking the user, a PST network upload, or a third-party tool are the usual alternatives.
• Move calendar and contacts separately. IMAP doesn’t carry them, so users import them into Outlook afterward.

How Do You Move Contacts, Calendars, and Tasks After an IMAP Migration?

IMAP only carries email, so contacts, calendars, and tasks need a separate route: user imports, PST export and import, or a third-party or managed migration that moves everything together. Decide this before cutover, not after users notice their calendars are empty.

• User self-service. Users can import their contacts and calendars into Outlook, which works for small teams and simple data.
• PST export and import. Export mailbox data to PST files and import them into Microsoft 365, which suits one-off or archive data.
• Third-party or managed migration. Many tools and partners can carry contacts, calendars, and tasks alongside mail in one pass, so users sign in to a complete mailbox on day one.

How Long Does an IMAP Migration Take?

Microsoft publishes no fixed duration, because speed depends on mailbox size, item counts, your source server’s connection limits, and your bandwidth, but you should always plan at least 72 hours before deleting a batch. The best way to estimate your own timeline is to measure it.

Run test batches with CSV files of equal size at similar times of day, compare their total running times, and use the results to size your real batches and choose how many simultaneous connections to allow. The table below is a planning estimate, not a Microsoft figure.

Mailboxes	Typical Project Length
Under 25	3 to 7 days, including the 72-hour window
25 to 250	1 to 2 weeks
250 to 1,000	2 to 4 weeks
1,000+	4 to 8 weeks or more

Very large mailboxes, strict connection limits, Gmail’s throttling, and a strict change freeze all push a project toward the top of its range.

How Do You Troubleshoot Common IMAP Migration Problems?

Most IMAP migration problems come down to a malformed CSV, wrong credentials, source server limits, or content the tool deliberately skips. Start with the per-mailbox status report in the Exchange admin center, which names the specific error.

Symptom	Likely Cause	What to Do
CSV won’t import	Headings misspelled, spaces added, or the wrong file type	Use exactly EmailAddress, UserName, and Password, and save as CSV
Mailbox fails immediately	Wrong EmailAddress, or no licensed mailbox yet	Confirm the Microsoft 365 address and that the mailbox exists
Authentication errors	Password changed, or Gmail app password missing	Refresh the credentials, or create an app password
Stuck on Syncing	Source throttling, low connection limits, or Google bandwidth limits	Raise limits, try smaller batches, or use another method
Messages or folders missing	Over 35 MB, over 500,000 items, a slash in the folder name, or retention policies	Rename folders, disable policies, and check the limits

What Are the Most Common IMAP Migration Mistakes?

The most common mistakes are skipping the test batch, getting the CSV wrong, and deleting the batch too early. Almost all of them are avoidable with the checklist above.

• Mailboxes not created or licensed first. The batch has nothing to migrate into.
• CSV heading errors. The headings must be exactly EmailAddress, UserName, and Password, with no spaces.
• Wrong EmailAddress values. That column must hold the Microsoft 365 address, not the source address.
• Password changes mid-migration. Before migration the batch fails, and after it new mail isn’t copied.
• Low connection limits. The source server throttles the migration, and a single admin account makes this worse.
• Policies left on. Retention and archive rules create false “missing” items.
• Deleting the batch too early. Mail sent to the old mailboxes after that point never arrives.
• Forgetting slash folders and large messages. They are silently left behind.
• Forgetting contacts and calendars. Users find out on day one.
• Skipping user communication. People don’t know their password, folders, or sign-in changed.

What Should You Do After the Migration?

After the batch is deleted, confirm every user can sign in and send mail, move their devices to Microsoft 365, and retire the old system only once you’re sure nothing is missing. A short welcome message helps more than most teams expect.

• First sign-in. Users sign in with their temporary password, set a new one, and choose the correct time zone, which keeps calendar and mail settings right.
• Reconnect devices. Remove the old IMAP account from Outlook and phones, and add the Microsoft 365 profile.
• Finish the DNS records. Add the records Microsoft 365 lists for mail flow and client setup, not only the MX record.
• Share old-system access if needed. If users still need the old system, give them its new passwords.
• Decommission carefully. Keep a backup, confirm nothing is missing, then shut down the source and remove access.
• Send a welcome note. Tell users how to sign in and where to find their mail, calendar, and files.

Built-In Tool, Third-Party Tool, or Managed Migration: Which Do You Need?

The built-in IMAP tool is enough for straightforward, email-only moves that you have time to monitor, while third-party tools and managed migrations make sense when you need more than email or can’t afford errors.

What You Need	Built-In IMAP Tool	Third-Party Tool	Managed Migration
Email	Yes	Yes	Yes
Contacts, calendars, tasks	No	Varies by tool	Yes, planned in
Who does the work	Your admins	Your admins, in the tool	Apps4Rent engineers
Best for	Small, simple, email-only moves	Specific feature gaps	Larger, riskier, or time-boxed projects

Migrating from a Specific Provider?

Many hosts have quirks worth knowing before you build your CSV. We have provider-specific guides for GoDaddy email, Rackspace, Namecheap, Network Solutions, Zoho Mail, and AWS WorkMail.

How Does Apps4Rent Help with IMAP Migration?

Apps4Rent plans and runs the full IMAP migration, from mailbox preparation and test batches to cutover and validation, and carries the contacts, calendars, and tasks that the built-in tool leaves behind. We have run migrations of this kind for organizations ranging from a 200-mailbox Gmail environment to a 350-mailbox financial firm. For the wider options, see our migration services.

• Environment review. Your source server, connection limits, policies, and mailbox sizes checked before anything syncs.
• CSV, endpoint, and test batch. Built and verified before your real users are involved.
• Credentials and cutover. Admin-credential migration where your server supports it, and a managed MX switch with the 72-hour window respected.
• Everything beyond email. Contacts, calendars, and tasks moved so users arrive to a complete mailbox.
• Validation and support. Results checked before the old system is retired, with 24/7 phone, chat, and email support.

FAQs About IMAP Migration to Office 365

1. How do I migrate IMAP email to Office 365?

   Create and license your users in Microsoft 365, build a CSV with the headings EmailAddress, UserName, and Password, create an IMAP migration endpoint in the Exchange admin center, then create and start an IMAP migration batch. After mail has synced, point your MX record at Microsoft 365, wait at least 72 hours, and delete the batch.

2. What is the IMAP migration tool for Office 365?

   It is the migration batch feature in the Exchange admin center that connects to an IMAP-enabled mail server through a migration endpoint and copies messages into existing Exchange Online mailboxes. Third-party tools also exist and can add features such as calendar and contact migration.

3. What does IMAP migration to Office 365 not migrate?

   It does not migrate contacts, calendar items, or tasks. It migrates only email from the inbox and other mail folders, up to 500,000 items per mailbox from newest to oldest, skips messages larger than 35 MB, and does not migrate folders with a forward slash in the name.

4. How do I move contacts and calendars after an IMAP migration?

   Users can import their own contacts and calendars into Outlook, or you can use PST export and import or a third-party or managed migration that carries everything in one pass. IMAP itself only carries email.

5. How many mailboxes can I migrate in one IMAP batch?

   A CSV file for an IMAP migration can hold up to 50,000 rows and be as large as 10 MB. In practice, Microsoft recommends running small test batches first and migrating in several smaller batches, so you can measure timing and limit the impact on your source server.

6. How long should I leave an IMAP migration batch running?

   At least 72 hours before you delete it. Mailboxes synchronize once a day, and it can take up to 72 hours for other email systems to recognize a changed MX record. Deleting the batch stops synchronization.

7. What happens to new email after the initial IMAP migration?

   While the batch exists, mailboxes synchronize once a day, so mail that arrives on the source server is copied across. Once you delete the batch, email sent to the old mailboxes is not copied to Microsoft 365, so delete it only after all users are working exclusively in Microsoft 365.

8. Can I migrate Gmail to Office 365 with IMAP, and do I need an app password?

   Yes for consumer Gmail, using IMAP with an app password for each account, which requires 2-step verification. Google Workspace organizations should use Microsoft’s dedicated Google Workspace migration instead, which also moves calendar and contacts.

9. Do I need to reset user passwords for an IMAP migration?

   Not if your source server supports admin credentials, which let you migrate without knowing or resetting user passwords. Otherwise you need each mailbox password, and if users change their password before migration the migration fails.

10. When should I use a managed migration instead of the built-in IMAP tool?

    Consider a managed migration when you need contacts, calendars, or tasks moved too, when you have many mailboxes or large mailboxes, when your source has strict connection limits, or when you can’t afford a mistake during cutover.