Read time 5 minutes

Summary: Migrating IMAP mailboxes to Office 365 or Exchange can be challenging. To optimize IMAP migration performance, inform users about migration requirements, adjust server settings, avoid mailbox changes during migration, and run test migrations. While the manual method has limitations, third-party tools like Kernel IMAP to Office 365 offer more flexibility and no constraints. These tools enable efficient, selective, and bulk migration of IMAP mailboxes, improving the overall process.

Optimizing IMAP mailbox content is easy – you can go through the folders to check for unnecessary emails, blocked contacts, trash items, tasks, etc. But, when it comes to migrating IMAP mailboxes, it becomes a little complicated for users to manage the overall process. Although you can easily migrate IMAP mailboxes to Office 365 or Exchange with various techniques, not very method is feasible in every situation. So, you need a reliable method that offers easy and quick IMAP migration.

When undertaking the migration of IMAP mailbox data from an on-premises Exchange Server to Microsoft 365, it is essential for users to focus on optimizing the performance of the IMAP migration to ensure its success. Therefore, it is imperative to consider and implement the following tips to enhance the efficiency of the IMAP migration process.

IMAP To Office 365 Migration Optimization Tips

  1. Let your Users Know About the Migration
    Prior to commencing the manual mailbox migration process, it is advisable to proactively notify your users about the impending migration. Consider the following key points to effectively communicate with your users:

    • The emails larger than 35 MB cannot be migrated, so inform users to save larger emails and attachments on their computer or any other device.
    • Ask users to remove unnecessary emails from the on-premises mailboxes prior to the migration. It will help in reducing the migration data while improving the migration time.
    • Ask users to backup the mailboxes in case of disaster.
    • Folders with (/) or slash symbol cannot be migrated, so inform users to rename these folders if they want to migrate them.
  2. Allow Maximum Connections to your IMAP Server
    Typically, each email server and firewall is equipped with specific restrictions concerning individual users, connection parameters, and IP addresses. Therefore, it is essential to confirm that both the firewall and IMAP server have been configured to accommodate the highest permissible values for the following settings prior to commencing mailbox migration:

    • Connections to the IMAP server
    • Connections from a particular user (especially when you use a CSV file for the migration)
    • Connections from a single IP address
      Note: If your IMAP server is running with Exchange 2010, 2007, then you will have to increase the connection limits because, by default, they are low.
  3. Avoid deleting mailboxes and changing their SMTP addresses during migration
    During the migration process, an error may occur if a migrated mailbox cannot be located. To address this, it is recommended to complete the migration first and then initiate the deletion of the migration batch. Following this, you can proceed to delete mailboxes or modify the SMTP address for an Office 365 or on-premises mailbox that has been successfully migrated.
  4. Modify the DNS Time-to-live Settings on MX Record
    Prior to commencing the migration process, it’s essential to adjust the Time-to-Live (TTL) settings within the current MX record of your Domain Name System. By reducing the TTL interval, you ensure that once the migration of all mailboxes to your Office 365 email organization is complete and you update the MX record to point to the new destination, the propagation of the MX record will occur swiftly, thanks to the shortened TTL interval.
  5. Use the Administrator Account for Migrating Emails with the CSV File
    By employing this approach, you can proactively prevent synchronization errors arising from password modifications made on the user’s end. Additionally, it alleviates the need to acquire and update user passwords. However, it’s imperative to ensure that the administrator account possesses the essential permissions required for accessing the mailboxes earmarked for migration when employing this method.
  6. Run a Test Migration
    Before starting the complete migration, run a few small IMAP migrations to test the migration performance. It will help you to:

    • Verify the CSV file format
    • Test the migration endpoint that is used to connect to IMAP server
    • Check whether you can migrate email successfully with administrator credentials
    • Verify the optimal number of simultaneous connections to the IMAP server for minimizing the impact on the internet bandwidth
    • Check that excluded folders are not migrated to the cloud
    • Verify how much time would it take to migrate a batch of users
    • Analyze how long would it take to migrate all the mailboxes

To ensure a successful migration when moving IMAP mailboxes from Exchange Server to Microsoft 365, it’s crucial to carefully consider the following recommendations. Moreover, by perusing these suggestions, you can gain valuable insights into the potential drawbacks of the manual migration approach. Additional limitations of this method include:

  • Migrates only emails and not contacts, calendars, tasks, etc.
  • Migrates email of maximum size up to 35MB only
  • Migrates maximum 500,00 mailbox items only
  • Need to increase connections
  • Long staged migration process
  • Not for naïve users

So, it is advised to use a reliable third-party tool like Kernel IMAP to Office 365 for the complete and successful migration of IMAP mailboxes to Office 365 with full flexibility and no limitations.

The Kernel IMAP to Office 365 is a sophisticated solution with a wide range of features designed to streamline the migration process for users. This tool empowers you to efficiently migrate multiple IMAP mailboxes simultaneously. Furthermore, it offers the flexibility of selective data migration, granting you the ability to apply customized filters to your IMAP mailboxes prior to initiating the migration.


The migration of IMAP mailboxes to Office 365 has emerged as a critical necessity for numerous businesses. This shift to the cloud presents significant advantages in terms of flexibility, reliability, and security. However, prior to embarking on the mailbox migration, it is imperative to streamline and enhance the IMAP migration process to guarantee its seamless success. In this article, we delve into pivotal strategies aimed at optimizing the IMAP migration process. We also address the intricacies and constraints associated with manual migration, ultimately recommending the adoption of a professional IMAP to Office 365 migration tool.