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IMAP migration


Applies to: Exchange Online


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You can use the Exchange Administration Center (EAC) or the Exchange Management Shell to migrate the contents of users' mailboxes from an IMAP messaging system to their Exchange Online mailboxes. This provides organizations with an efficient way to move from an IMAP messaging system to an Office 365 email organization. IMAP migration supports the following IMAP servers:

  • Courier-IMAP

  • Cyrus

  • Dovecot


  • Microsoft Exchange (Exchange 2000 Server and later versions)

IMAP migrations can also be used by small and medium-sized businesses to migrate email from mailboxes on a hosted service provider.

To learn about and compare other options for migrating mailboxes to Exchange Online, see Mailbox migration to Exchange Online. Or, use the Cloud Only option in the Exchange 2013 Deployment Assistant to help select the best migration option for your organization.

For additional options for migrating email to Office 365, see Migrate email to Office 365.

How the IMAP migration process works

Here's a description of the IMAP migration process. It includes steps performed by an administrator and the migration process performed by Exchange Online. For preparation tasks and step-by-step instructions, see Migrate Email from an IMAP Server to Exchange Online Mailboxes.

  1. The administrator provisions new Exchange Online mailboxes by creating Office 365 user accounts and assigning Exchange Online user licenses. For more information, see Create user mailboxes in Exchange Online.

  2. The administrator creates a CSV file that contains a row for each user whose IMAP mailbox will be migrated.

  3. The administrator creates and starts an IMAP migration batch using the Migration dashboard in the EAC or using the Exchange Management Shell.

  4. After the administrator starts the migration batch, Exchange Online does the following:

    • Creates a migration request for each user listed in the CSV file. Each migration request contains the user name and encrypted password for the account in the IMAP messaging system.

    • Starts processing migration requests. This part of the process is called initial synchronization. For each user listed in the CSV file, messages from the IMAP mailbox are copied to the corresponding Exchange Online mailbox until all the mailbox data is migrated. Exchange Online processes up to N migration requests at one time, where N is the maximum number of concurrent migrations that the administrator specified when creating the migration endpoint used for the migration batch. By default, initial synchronization is performed on 20 mailboxes at a time until all mailboxes in the migration batch have been migrated.

    • Processes the migration requests for all the users listed in the CSV file.

  5. When initial synchronization is completed for a migration batch, Exchange Online sends a status email message to the administrator. This message lists the number of mailboxes that were successfully migrated and how many couldn’t be migrated. The message also includes links to migration statistics and error reports that contain more detailed information.

  6. After initial synchronization is complete for the migration batch, mailboxes in the IMAP messaging system and the corresponding Exchange Online mailboxes are synchronized every 24 hours. This is called incremental synchronization. This means that any new messages sent to the mailboxes on the IMAP server are copied to the corresponding Exchange Online mailboxes. By default, incremental synchronization is performed on 10 mailboxes at a time until all mailboxes in the migration batch are synchronized. Incremental synchronization continues until the administrator stops or deletes the migration batch.


    During incremental synchronization, deletions made in an IMAP mailbox are synchronized to the corresponding Exchange Online mailbox. This means that if a user deletes an item from an IMAP mailbox, it will be deleted from the Exchange Online mailbox.

  7. The administrator creates and starts additional migration batches, if necessary.

  8. After the administrator resolves any migration issues and all IMAP mailboxes have been successfully migrated, and users are using their Exchange Online mailboxes (and no longer using their mailboxes on the IMAP system), the administrator updates the organization's MX record to point to their Office 365 email organization. This causes email messages to be delivered directly to users' Exchange Online mailboxes.

  9. After the change to the MX record has been propagated and mail is flowing directly to Exchange Online mailboxes, the administrator deletes the IMAP migration batches. After the migration batch is deleted, mailboxes on the IMAP system are no longer synchronized with Exchange Online mailboxes.

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