Migrate from other messaging systems

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Because IMAP4 is so widely supported, it’s often an option you can use to import mailboxes from non-Exchange on-premises systems. You should also consider IMAP4 migrations if you have Exchange Server 2000 or earlier. You must prepare a list of mailboxes that you can then import from the source system by using either the Exchange admin center (EAC) or PowerShell.

Preparing for the migration

Before you can perform the migration, complete the following tasks:

  • Make sure that the DNS domain for your on-premises system is an accepted domain in Office 365.
  • Add mailboxes in Exchange Online for all the users you want to migrate.
  • Configure your on-premises firewall to accept IMAP4 connections on TCP port 143 or port 993 for SSL-protected connections.
  • Create a migration endpoint in Microsoft 365, as described in the previous Configure migration endpoints unit. For the endpoint type, choose IMAP, and then enter the name of the on-premises IMAP server.

You must also create a comma-separated text file, which lists a mailbox to import on each line and has three columns:

  1. EmailAddress. In this column, enter the user ID for the mailbox in Exchange Online.
  2. UserName. In this column, enter the username for the mailbox in the on-premises email system.
  3. Password. In this column, enter the password for the mailbox in the on-premises email system.

You can use Microsoft Excel or any text editor to create these files:

Mailbox import text file

In Excel, to save the list as a .csv file, select File > Export > Change File Type. Under Other File Types, select CSV (Comma delimited) (*.csv), and then select Save As.

Executing the migration in the EAC

To create and run a migration batch that uses the IMAP protocol, follow these steps:

  1. In the EAC, in the dashboard under Recipients, select Migration.

  2. Select +, and then click Migrate to Exchange Online.

  3. On the Select a migration type page, select IMAP migration, and then click Next.

    Select the IMAP migration type

  4. On the Select the users page, click Browse, select the .csv file you created, and then select Next.

    Select the .csv file on the users page

  5. On the IMAP migration configuration page, enter the name of the IMAP server, select the authentication and encryption option that the IMAP server supports, and then select Next. Exchange Online attempts to connect to the IMAP server.

    Enter the IMAP server name on the IMAP migration configuration page

  6. Select the migration endpoint that you created for the IMAP server.

  7. On the Move configuration page, in the Name text box, type a descriptive name for this migration batch, and then click Next.

  8. On the Start the batch page, click Automatically start the batch. Exchange Online creates the migration batch and starts to import mailboxes.

Executing the migration in PowerShell

If you prefer to use a command-line interface, or you want to include the migration in a script, you can execute a migration batch by using PowerShell. Before you start, make sure you’ve completed the tasks in the Preparing for the migration section above.

To create and start a migration batch, use this command:

New-MigrationBatch -Name IMAPImport 
   -SourceEndpoint IMAPonpremises 
   -CSVData ([System.IO.File]::ReadAllBytes("C:\mailboxes.csv")) 
   -AutoStart

While the batch is running, you can check its progress by using this command:

Get-MigrationBatch -Identity IMAPImport | Format-List Status

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