Here are the steps required in sequence to migrate an IMAP domain to Exchange Online.
- Add the domain to your Microsoft 365 tenant. You don’t have to complete the mail server setup.
- Add domain users to Microsoft 365. Each user must have a Microsoft 365 Business Basic, Standard, or Premium license
- Prepare the csv file for migration, separated by commas. In the first line put EmailAddress, UserName, Password. In the following lines the data: “EmailAddress is the Microsoft account,” UserName “is the imap server account and” Password “is the imap server password
example of csv
EmailAddress,UserName,Password firstname.lastname@example.org,contoso\terry.adams,1091990 email@example.com,contoso\ann.beebe,2111991 firstname.lastname@example.org,contoso\paul.cannon,3281986
- Log in as an administrator in Microsoft 365 and go to the Exchange admin center. (Note: this guide is for the “classic” Exchange administration interface. Select “recipients” on the left; select “migration” at the top.
- At the center of the page there is a button with three dots: …. Selecting it, the endpoint is inserted, that is the Imap source server. In the next window add the new endpoint (IMAP).
- Create a new migration. launch the migration
- once the migration is complete, in the tenant, you can finish configuring the domain for what concerns the mail server, following the instructions on the tenant and changing your dns
The Rules of Migration
You can put all users in a migration. When a migration ends in error, you can delete a user from it and put the same user in another migration. You can have multiple migrations at the same time but the same user cannot exist in more than one migration. Migration can exist for up to 60 days.
It is not a migration
In reality, Microsoft does a more sophisticated operation than a “trivial” migration: it makes a sync. Synchronize entire imap mailbox to Exchange mailbox in one direction (from imap to exchange). It is sophisticated but less effective than a normal migration: it is not in real time but after 24/30 hours. So if you want to replace the mail server, users would lose at least 24 hours of email.
On Outlook clients, you can add the new Exchange account online. It will be the same as the old mailbox, but will be managed by Exchange. For a while you will then have 2 mailboxes that manage the same mail but on different servers: one is the old imap server, the other one the new Exchange server. When the migration is finished and you have also moved the mx records on the dns, you can delete the old mailbox. Before doing this, however, you must also memorize the contacts and the calendar from the “old” to the “new”:
Contacts: select all contacts, right click, select “move” and then “copy to folder …”, Exchange mailbox contacts.
Calendar: To move appointments between 2 calendars: both calendars and drag appointments from old to new.
Problems in migration
If you have any problem you can investigate using PowerShell. First install ExchangeOnlineManagement.
Connect to the tenant:
Connect-ExchangeOnline -UserPrincipalName <your Admin Username>
The password request screen appears.
List of all endpoints in the tenant
Test-MigrationServerAvailability -Endpoint <Identity of the endpoint from above>
view sync configuration of single user
Get-SyncRequest -Mailbox <user>
esport migration result for a user
Get-MigrationUserStatistics <user> -IncludeSkippedItems -IncludeReport -DiagnosticInfo "showtimeslots, showtimeline, verbose" | Export-Clixml C:\temp\MigMyUser.xml
Exchange mailboxes have a 35MB limit. If you have to move something bigger during the migration you have to change this limit.
Set-Mailbox -Identity <user> -MaxReceiveSize 150MB
- Domain on tenant : https://docs.microsoft.com/en-us/microsoft-365/admin/setup/add-domain?view=o365-worldwide
- file csv : https://docs.microsoft.com/en-us/exchange/mailbox-migration/migrating-imap-mailboxes/csv-files-for-imap-migrations
- migration : https://docs.microsoft.com/en-us/Exchange/mailbox-migration/migrating-imap-mailboxes/imap-migration-in-the-admin-center?redirectSourcePath=%252fen-us%252farticle%252fIMAP-migration-in-the-Office-365-admin-center-4682f2e4-f720-4868-91ab-207f5b0c325d