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Summary: This article discusses common issues encountered when migrating IMAP mailboxes to various destinations, especially in the context of Office 365 Admin Center. The issues covered include problems during the migration batch start, such as duplicate emails, full mailboxes, login troubles, and recipient not found errors. Additionally, the article addresses issues preventing the migration batch from starting, like batch name conflicts, CSV file errors, server connection problems, and slow mailbox migrations. Troubleshooting solutions are provided for each problem. The article concludes by suggesting a professional migration tool called Kernel IMAP to Office 365 for more efficient and automated IMAP mailbox migrations. Manual methods, while possible, can be time-consuming and technically complex.

Check this guide to know all troubleshooting measures to handle the issues in migrating IMAP mailboxes to any destination.

IMAP migration in Office 365 Admin Center is a widely used method, but it frequently encounters errors during the migration process. In this discussion, we will explore the most common errors that arise in this complex migration procedure and provide straightforward troubleshooting solutions to address them.

Common Issues with IMAP Mailboxes Migration

  • Issues on starting the migration batch
    If able to start the migration batch using the play option, the following issues can occur

    1. Duplicate emails
    2. The mailbox is full
    3. Trouble signing into the account
    4. A recipient wasn’t found for user@contoso.com on the target

    If unable to create/start the migration batch, the following issues can occur

    1. Batch <name> already exists
    2. Errors with CSV files
    3. Connection to the server couldn’t be completed
    4. The connection to the server servername.contoso.com timed out
  • Mailboxes migration is running too slow

Troubleshooting Solutions

We will go through the troubleshooting solutions in all the situations above.

Migration Batch Issues When It Gets Started

To initiate the migration batch, begin by resuming or commencing it anew. Simply select the target migration batch and click on the play icon. Confirm the start of the migration by clicking ‘Yes.’ Be aware that the process may encounter various issues, prompting us to delve into troubleshooting solutions, which we will discuss shortly.

  • Duplicate emails

    When the migration process overlooks the inclusion of the Gmail folder in the Exclude folders section, it can be resolved by taking two specific actions.

    • Selecting the Gmail folder under the Exclude folders section while creating the migration batch again.
    • Removing labels from emails to migrate in the source mailbox – First, filter the emails with labels, select those emails, and then the Labels Clear the selected checkboxes against all labels and click Apply option finally.
  • The mailbox is full

    It is an error saying, The mailbox is full. Please free up some space so that the email can be downloaded.This error generally occurs when the storage limit for Microsoft 365 mailbox is exceeded. It happens due to duplicate emails.

    Users can try deleting the existing migration batch by navigating to recipients>migration, selecting the existing migration batch, clicking the Stop option, removing the labels from emails as done earlier, and deleting duplicate emails and Gmail folder from the Microsoft 365 mailbox.

    Also, make sure to remove labels from emails before migration and exclude the Gmail folder while migrating it to the cloud destination.

  • Trouble while signing in to the account

    Once the migration batch is initiated, users may come across the following error message: “We encountered sign-in issues with this account. Please ensure that you have entered the correct username and password.” This error typically arises when the CSV file utilized for the migration contains inaccurate usernames and passwords for IMAP accounts.

    Users have two options: they can either input the correct passwords corresponding to the accounts listed in the CSV file or utilize administrator credentials that grant them the necessary access rights, such as Full Access or Receive As permissions, to all user mailboxes within that account. As an illustration, consider a user named “x” whose entry in the CSV file would appear as follows.



    If the above details are already correct, check if the IMAP mailbox exists or not by visiting the recipients>mailboxes location in the Exchange Admin.

    In order to ensure the successful execution of the migration batch, it is imperative that IMAP access for user mailboxes slated for migration is enabled through the IMAP settings within the respective application, such as Gmail.

  • A recipient wasn’t found for user@exmaple.com on the target

    This error can be resolved by the user through the creation of a new user mailbox within the designated Microsoft 365 account. Utilizing the provided proxy address, the user can then reinitiate the migration batch.

    If the issue persists, begin by inspecting the CSV file for the IMAP migration’s first column, which contains email addresses (EmailAddress), ensuring that the entries have valid proxy addresses for the target Microsoft 365 account mailbox. After confirming the accuracy of these entries, proceed to restart the migration batch and monitor the process for smooth execution.

    Migration Batch Issues When It Does Not Get Start

    If you have attempted to start the migration batch, but it does not get started rather throw errors, then you need to perform some different troubleshooting solutions.

  • Batch <name>already exists

    A common mistake occurs when you’ve chosen a migration batch name identical to an existing one. In such cases, you must either generate a migration batch with a unique name or eliminate the pre-existing migration batch. To delete the existing batch, go to the Exchange Admin Center, navigate to recipients > migration, select the batch you want to remove, and click the “Delete” option. Afterward, you can establish a fresh migration batch with the same name and continue with the migration process.

  • Errors with CSV files

    Some common CSV errors responsible for the issue in starting the IMAP migration batch along with troubleshooting solutions are –

    • The CSV file contains more than 50,000 rows – This represents the maximum row limit for the CSV file designed for migration purposes. Therefore, it is advisable to employ multiple files or process data in batches when migrating more than 50,000 users.
    • Column <column name> is missing from the CSV file – If any of the columns in the standard CSV file format – specifically, Email Address, Username, and Password – are missing, an error will be triggered. To resolve this, please review your CSV file and ensure that all the necessary details for these columns are included. Once the missing column information has been added, you can proceed to retry the migration batch.
    • Upload file can’t be empty – The user has made a critical error by either omitting the correct CSV file or submitting an empty file lacking essential details. Consequently, it is imperative to conduct a thorough review and ensure that all necessary information is added to the CSV file prior to its inclusion.
    • Row X has the wrong number of columns – The error occurs when column information is absent in one or more rows of the CSV file. While this is a common mistake, it can be rectified by editing and re-uploading the CSV file.

    Connection to the server could not be completed

    This error arises when specifying the IMAP server’s Fully Qualified Domain Name (FQDN) in various scenarios. It is essential to employ the appropriate troubleshooting solution for each specific situation.

    While creating a migration batch with no endpoints – Here is the process to specify the public FQDN correctly.

    1. Log in to the Exchange Admin Center of the account used for migration (with administrator credentials).
    2. Go to recipients>migration.
    3. Click on the plus icon to select the Migrate to Exchange Online
    4. Choose IMAP migration and click Next.
    5. Then click the Choose file option, browse, and add the CSV file. Click next.
    6. Then enter the public and resolvable FQDN (example.com) and click next to proceed with creating and running the migration batch finally.

     While creating the migration batch with the existing endpoint, follow this process to determine the public FQDN.

    1. On the Exchange Admin Center, go to recipients>migration.
    2. Click the 3 dots or More option and then select the Migration endpoints
    3. Select the migration endpoint
    4. Then check the FQDN of the IMAP Serer at the bottom of the page.
    5. Recreate the migration endpoint if it is incorrect or not publicly resolvable.

    The connection to the server <servername.example.com> timed out

    It happens when –

    • Firewall does not allow IMAP connections to IMAP Server via Microsoft datacenters – For that, check the Firewall settings, install the client to verify correct flow to IMAP Server, etc.
    • The IMAP service application is not running – Ensure the IMAP service like Exchange, Notes, etc. is running in its source environment. Run msc in the Exchange Server and confirm if Microsoft Exchange IMAP4 service is started or not.

    Mailbox migration is running too slow

    Please find the below pointers to deal with the situation where your mailboxes under migration are moving at a very slow pace.

    • Increase the default number of simultaneous moves from 20 to a larger number by navigating to recipients>migration>More>migration endpoints, double-clicking the endpoint to move to Max concurrent migrations, and changing its value by clicking Save.
    • Check the migration report by visiting the View details section of the selected migration batch. It would tell the status of the mailboxes under migration, whether Syncing or Items Synced.
    • Increase connection limits to the IMAP Server
    • Check server and IDS settings
    • Delete existing migration batch and recreate it

    We have now developed effective troubleshooting solutions for IMAP mailbox migration to the cloud platform. However, navigating the myriad challenges during native IMAP mailbox migration can be quite testing and demands considerable patience. To address this, we recommend the use of the highly efficient and automated professional tool called Kernel IMAP to Office 365 migration. This tool excels in delivering precise IMAP Server mailbox migration, offering a multitude of options and an intuitive graphical user interface to streamline the process.


    During the migration of IMAP mailboxes to Office 365 using manual methods, email users may face a lot of issues. Though there are some fixes for all these issues, they may consume a lot of time and effort. Also, these methods are technically complex. So, it is recommended to use a professional solution for IMAP to Office 365 migrations.