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Key Challenges with M365 Cross-Tenant Migrations

Updated: Dec 10, 2023

Facilitating seamless migration of Microsoft 365 (M365) data across tenants requires firsthand experience as well as excellent knowledge of the capabilities of the designated migration tool. Besides the obvious functional requirements, the objective of such projects is to ensure smooth transition of M365 services and associated data from source to target while proactively mitigating potential challenges inherent in cross-tenant migrations and ensuring clarity and visibility on migrated objects at every stage of the migration.


Migrating from one M365 tenant to another

Even though M365 cross-tenant migrations could be defined as collection of repetitive and predictable tasks, every project differs because of the many differrent ways clients use cloud services to achieve their business goals. Thus, the technical challenges cannot always be predicted, and we want to share some of the issues you might face. Below is a list of problems you might experience when delivering such projects.


Mail Loops & Unwanted Notifications

Migration of transport rules after a domain cutover involves ensuring smooth transition of email services from one domain to another while maintaining the transport rules’ functionality. Failure to do so may result in mail loops, drops or other mail flow related issues. Additionally, users could receive unwanted notifications during the mail forwarding reconfiguration.


Mail loops can happen as a result of missed/skipped transport rules which in turn could cause incoming mails to bounce back to an external mail hygiene system (e.g., Mimecast). To avoid such cases, make sure to have an up-to-date export of all transport rules and their purpose before making changes to the mail flow. Unwanted notifications can be easily avoided with proper planning, validation, and simple reconfiguration.


New Identities & Passwords

Due to the way how the cloud identity service is designed (i.e. Entra ID), when migrating between tenants users have to use new passwords and their old UPNs in the destination tenant. This process is sometime confusing to them and might lead to service desk escalations during the first few hours and days after the switch to the new tenant.


The key to success here is to plan and deliver proper sessions outlining the process. Also, work with the client and distribute an easy-to-understand communication at specific intervals and before a change is made.


Incomplete Migration of SharePoint Data

One prominent challenge is the complete migration all SharePoint files. Depending on the client’s usage of SharePoint and the known limitations of the migration tool you might end up having files which were not transferred successfully. This could lead to potential data gaps, inconsistencies, or a major business impact.


It is not possible to avoid such issues entirely and if you happen to experience them make sure to understand the nature of the problem, i.e., is it related to a known limitation of the migration tool (or M365) or there is another root cause. You might have to work with the migration tool vendor by implementing a targeted solution with their help. A due diligent review and adjustment of migration settings, combined with a persistent and persuasive approach will always help achieving better results and a smoother transition.


Downtime & User Impact

At some point, the migration process usually requires service downtime. This could be mitigated at the cost of the end user experience, e.g., asking users to sign-in with onmicrosoft.com UPNs until the vanity domains are fully moved to the new tenant. Finding the balance between acceptable downtime and end user discomfort is very much specific for each client. Minimising this downtime and optimising the migration timeline are essential to reduce disruptions to business operations. Here are some additional guidelines:

  • Maintain clear and timely communication throughout the project by apprising users of the impending changes, potential downtime, and actions required on their part.

  • Outline procedures for device reconfiguration by providing step-by-step instructions to IT personnel. The instructions should contain relevant use cases, e.g., for Azure joined devices and Azure registered devices. Put an emphasis on the IT personnel readiness and explain the importance of following these procedures to guarantee uninterrupted access to essential applications and data post-migration.


Data Mapping & Structure

Mapping data structures between source and destination tenants is a complex task which requires overcoming challenges. Ensuring that data integrity is maintained and relevant to the new environment requires meticulous planning and execution. A general guideline here is to enforce constant alignment with the business, from the start to the end of the project.


Key Takeaways

During tenant cutover, when switching from one vanity domain to another, it is crucial to carefully review and reconfigure transport rules to align them with the new environment. This process can be challenging due to potential differences in email systems, addressing schemes, or security settings between the old and new tenants.


Communication is the key to success. It must be executed timely, and the end users’ feedback validated thoroughly in order to minimise the potential impact on operations after users are moved to the new tenant. It might sound simple and obvious, but it is also crucial that end users know their new credentials.


Conduct a comprehensive evaluation of the migrated data to ensure its integrity, completeness, and accuracy during initial data sync. Verify that all files, documents, and associated metadata have successfully transitioned to the new M365 tenant without any loss or corruption.


Ensure that devices are appropriately reconfigured and that users have regained access to essential applications and data. This step allows for the identification of any residual issues that may require further attention.


Establish a continuous improvement plan based on the insights gained from the post-migration evaluation. Identify areas for enhancement, implement necessary adjustments, and document lessons learned.

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