Migrating On-Premises Email to Office 365: Limitations, Prerequisites and Best Practices

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Office 365 can host on-premises messaging environmentS, including IMAP mailboxes, through different migration methods. IMAP migration, cutover migration, staged migration, hybrid deployment, etc.

These onboarding methods support the migration of email, calendar and contact data from the on-premises messaging platform to Office 365. Depending upon the on-premises setup and migration method, there could be specific limitations, prerequisites and best practices that you should consider for a smooth migration. This article outlines these critical considerations for migrating an on-premises email organization to Office 365.  

Office 365 Email Migration Methods: Limitations

IMAP Migration: You can migrate mailboxes from any IMAP email system to Office 365, however, there are a few limitations of IMAP migration:

  1. IMAP migration can migrate only the mail items from inbox or another folder. It cannot migrate contacts, calendar or tasks. You would need to migrate these manually at a later stage
  2. It allows migration of up to 500,000 items from a mailbox with a maximum size limit of 35 MB per email

Cutover Migration: The cutover method is used when you migrate the contents of all user mailboxes at once. Here are the limitations:

  1. The cutover method can migrate a maximum of 2000 mailboxes to Office 365, although the recommended limit is just 150 mailboxes
  2. You must changeover the domain immediately, and some emails might be lost in transit until the DNS record is fully changed.

Staged Migration: This method involves the migration of mailboxes to Office 365 in stages, over a period of time. It is recommended when you need to migrate many mailboxes with big sizes. Its limitations include:

The staged method can migrate only the user mailboxes and resource mailboxes. It cannot migrate distribution groups, contacts and mail-enabled users

It cannot migrate Out of Office messages along with the user mailboxes

Hybrid Method: This method involves interconnecting Office 365 and the local Exchange Server. It is ideal when you have a complex setup and you need to migrate a lot of mailboxes and data over a longer duration. Its limitations include:

  1. Hybrid setup requires more administrative effort to maintain
  2. It involves several steps and complex setup to initiate the configuration

Office 365 Email Migration: Prerequisites and Best Practices

Migration Aspect

Prerequisites/Best Practice

IMAP Server Connection Limit

Increase the connection limits to IMAP server: A higher threshold is required to allow more numbers of connections for the different settings such as the connections to IMAP server, connections per user and connections from an IP address.

Refer to the Microsoft documentation for IMAP4 connection limits for Exchange 2013.

Time-to-Live (TTL)

Reduce the DNS Time-to-Live setting: Update the TTL setting on your MX record to a shorter interval, let’s say 2700 seconds (or 45 mins). This change will speed up the propagation of the updated MX record after migration.

Migration Testing

Perform a test migration: For the IMAP method, migrate a few smaller batches of the mailboxes to test and verify the crucial aspects such as CSV file format, migration endpoint, optimal IMAP connections, the average time for a batch migration, etc.

Credential Synchronization

Use administrator credentials in the CSV file: Using admin credentials will eliminate the chances of synchronization errors otherwise caused due to changing of the on-premises account password by the user. 

Mailbox Deletion

Avoid deleting a mailbox during migration: Don’t delete a mailbox or update its SMTP address while the migration is in progress. Otherwise, the system will report an error, disrupting the migration process.

Outlook Anywhere

Configure Outlook Anywhere on Exchange Server: Cutover and staged migration methods rely on Outlook Anywhere to establish a connection with the on-premises Exchange Server. So, set up Outlook Anywhere before initiating the migration process. 

SSL Certificate

Use a certificate from a trusted certification authority (CA): Installing a trustworthy certificate with Outlook Anywhere is essential for cutover and staged migration. Refer to the Microsoft documentation for configuring SSL certificates with Exchange 2013.  


Assign the necessary permissions: For the cutover method, assign Full Access permission to the migration administrator for each on-premises mailbox, or assign Receive As permission on the on-premises database that stores the mailboxes.

For a staged method, assign Full Access permission for each on-premises mailbox and assign Write Property permission to modify the Target Address property.

Refer to the Microsoft documentation for assigning Exchange permissions.

Account Synchronization

Synchronize on-premises account with Office 365: For staged migration, synchronize the user accounts on the on-premises AD domain with Office 365 by using Azure Active Directory sync.

Mailbox Database

Consider the Legacy Mailboxes: This is a crucial aspect to attain ‘comprehensive’ migration to Office 365. While the standard methods will migrate the mailboxes of the current databases, you also need to consider the archived databases associated with a legacy server.

Migrating the legacy mailboxes of Exchange would require mounting of the EDB, which is effort and cost-intensive and may not be feasible if, for instance, the AD configuration is lost or the support has ended.

A third party EDB to PST converter can solve this problem by opening the EDB file directly, without having an Exchange Server, and converting the mailboxes into PST file format. Stellar Converter for EDB is one such tool that converts EDB into PST and can also export the PST files to Office 365.

Migrating an email organization to Office 365 is faster and easier as compared to upgrading an on-premises messaging platform, apart from the cost involved in licensing, resources and support. The availability of well-documented migration methods facilitates a targeted route for onboarding on-premises, IMAP and other mailboxes to Office 365. However, there are a few limitations, prerequisites and best practices with these methods. Having a ‘big picture’ view of these aspects can facilitate a smooth migration of your email organization to Office 365. Further, tools like Stellar Converter for EDB can facilitate the process by extracting the mailboxes from any EDB file without needing Exchange Server and export the mailboxes directly to the Office 365 tenant.

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