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Why it happens

The UserName or Password value in the migration CSV file is wrong, IMAP access is turned off for the user, or the source account cannot be accessed.

Where you see it, and what it affects

The error shows for each affected user in an IMAP migration batch to Exchange Online. Microsoft 365 sends an email titled E-mail migration batch "Batch name" has finished - with errors, with a link to download the error report. You can also see it in the Exchange admin center under Migration, then the batch and View details. Some third-party IMAP systems do not accept admin (super-user) credentials, so check the CSV format for your source.

How to fix it

1. 1

   Step 1

   Correct the user name and password in the CSV file. With an admin (super-user) account, the user name format includes the admin and the mailbox, for example contoso/mailadmin/usera.

2. 2

   Step 2

   Check the mailbox exists on the source server and that IMAP access is enabled for the user. For Gmail, IMAP is turned on in Google settings.

3. 3

   Step 3

   If you use an admin account, give it Full Access on each mailbox, Receive As on the mailbox database, or membership of Domain Admins.

4. 4

   Step 4

   Retry the migration from Recipients > Migration in the Exchange admin center. See create a migration batch.

EmailAddress,UserName,Password
usera@contoso.com,contoso/usera,P@ssw0rd1
userb@contoso.com,contoso/mailadmin/userb,P@ssw0rd

Check that it is fixed

• In the Exchange admin center, go to Migration, select the batch and select View details. The user's Status should show Syncing.
• Refresh the report after a couple of hours. The Items Synced number should be going up.

Sources

Full guide: How to migrate IMAP email to Microsoft 365 →