This article provides you with with information on Migrating your IMAP mailboxes to Office 365

You can migrate the contents of user mailboxes from your source email system to Office 365.

Things to consider

Here are a few limitations to be aware of:

  • You can only migrate items in a user's inbox or other mail folders. This type of migration doesn't migrate contacts, calendar items, or tasks.

  • You can migrate a maximum of 500,000 items from a user's mailbox (emails are migrated from newest to oldest).

  • The biggest email you can migrate is 35 MB.

  • If you limited the connections to your source email system, it's a good idea to increase them to improve migration performance. Common connection limits include client/server total connections, per-user connections, and IP address connections on either the server or the firewall.

How does IMAP migration work?

The main steps you perform for an IMAP email migration are shown in the following illustration.

Process for an IMAP email migration

  • First you have to create your users in Office 365 and assign licenses to them. The mailboxes have to exist in Office 365 to use IMAP migration.
  • Prepare your IMAP source email system and get the information you need to migrate. If you plan to migrate your domain to Office 365, verify that you own your domain.
  • Depending on which type of email service you are migrating from, you might need to configure some settings or simply record the name of your email server or service to use later.
  • Communicate changes to email users.
  • It's a good idea to let users know about the email migration and how it impacts them. Give users information about what tasks need to be done before, during, and after migration.
  • Set up admin credentials or get or reset user email passwords.
  • To perform the migration, you need an administrator account that has permissions, or the username and password to each mailbox.
  • If you are using the steps described in Migrate Google Apps mailboxes to Office 365 or Migrate other types of IMAP mailboxes to Office 365, you will create a list of mailboxes to migrate (CSV file). These migrations instructions start from the Exchange admin center, and you will need to create a CSV file that lists the email addresses, usernames, and passwords for the mailboxes you want to migrate.
  • You can also use the migrations page or setup instructions in the Admin center preview to migrate from IMAP systems such as Gmail, or These steps are the best if you plan to migrate mail for only a few users (less than 50). If you are migrating mail for more users it is easier to use a CSV file to enter all the information for the accounts.
  • Connect Office 365 to email system.
  • To migrate email successfully, Office 365 needs to connect and communicate with the source email system. To do this, Office 365 uses a migration endpoint, the settings that are used to create the connection.
  • Migrate mailboxes and then verify the migration.
  • To migrate mailboxes, you create a migration batch, and then start the migration. After the migration batch is run, verify that the email was migrated successfully.
  • Optimize email settings (optional).
  • There are some settings you can configure so that it doesn't take as long for email to start showing up in your new Office 365 mailboxes. See Tips for optimizing IMAP migrations.
  • Begin routing email to Office 365.
  • You need to change a DNS record called an MX record so that your email system can start routing mail to Office 365.
  • Verify routing and then stop email synchronization.
  • After you verify that all email is being routed to Office 365, you can delete the migration batch to stop the synchronization between your source email system and Office 365.
  • Send a welcome letter to users.
  • Let your users know about Office 365 and how to sign in to their new mailboxes.

TIP: For more information regarding Office 365 Migration tool please see Microsoft's website here.