IMAP Migration:
This migration method is used when a non-Exchange mail system is involved. IMAP migrations are capable of moving e-mail messages only, however, contacts, calendars, and tasks are lost during the process. This method is sometimes used as a shortcut to move from the older versions of Exchange Server (Exchange 2000 & newer supported) to the cloud.

• First you have to create your users in Microsoft 365 or Office 365 and assign licenses to them. The mailboxes have to exist in Microsoft 365 or 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 Microsoft 365 or Office 365, verify that you own your domain with your domain registrar.
• Depending on which type of email service you are migrating from, you might need to configure some settings or record the name of your email server or service to use later. You also need to verify your domain in your domain registry system if you have a custom domain.
• Communicate changes to 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 Microsoft 365 or Office 365 or Migrate other types of IMAP mailboxes to Microsoft 365 or 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, Hotmail.com, or Outlook.com. 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 Microsoft 365 or Office 365 to the source email system.
• To migrate email successfully, Microsoft 365 or Office 365 needs to connect and communicate with the source email system. To do this, Microsoft 365 or 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 Microsoft 365 or Office 365 mailboxes. See Tips for optimizing IMAP migrations.
• Begin routing email to Microsoft 365 or 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 Microsoft 365 or Office 365, you can delete the migration batch to stop the synchronization between your source email system and Microsoft 365 or Office 365.
• Send a welcome letter to users.

Hope this helps,
Jimmy Wick