4,000 M365 Mailboxes Migrated in One Weekend


The Business Challenge

As part of a demerger, one of our customers was required to migrate their M365 services, domain name and user accounts from a legacy tenant belonging to the parent company to a brand new M365 tenant.

Planning to Migrate 4000 Mailboxes In One Weekend

The timeline was tight with a commercially immovable deadline and we were given three months from start to finish to move 4000 user mailboxes, associated shared mailboxes, Teams data, SharePoint data and OneDrive data. The customer provides critical services 24×7 meaning the disruption and any outage had to be minimised and carefully controlled.

We carefully selected our approach to migration with minimising the impact to end users foremost in our minds. We worked with our partner Binary Tree and used a suite of Binary Tree tools to pre-synchronise data and manage and control the migration. Before starting the synchronisation, our security team hardened the new M365 tenant by applying our best practice EOP configuration. The initial synchronisation started one month before the planned cut-over date. Dummy accounts were created to validate the process and fine tune the migration configuration.

In parallel, our project managers worked closely with the customer’s business departments to agree the best date and window to perform the migration and built a detailed run book of technical and communications events to be followed including how to roll back, and when to start to ensure that the service required by the business was available as agreed. The IT Naturally team drafted all the advanced communications and instructions for users as well as a series of carefully drafted update communications to be used to inform progress during the migration.

A change freeze one week before the change was agreed between IT Naturally’s service management team and the customer’s business.

Controlling the Change

The actual tenant migration was divided into two parts, the first being the final synchronisation and cutover of M365 services including email, and the second being the migration of the domain name.

The change was governed by strict entry criteria that were developed over time and captured in the start of the run book. The customer’s business departments as well as its IT team were required to accept that the change could start, validating that all technical and business preparations had been made and were complete. Once the change was underway, we used Microsoft Teams messaging to communicate between all technical teams and business representatives to track and monitor progress, carefully ticking of task after task from on the run book. Our Service Desk watched over each step around the clock coordinating all activities as we went. Meetings were periodically set up in advance with people’s time reserved just in case there were any problems. The total duration of the change was 60 hours and starting on a Friday lunchtime the migration of 4000 users happened over a single weekend.

Customer Outcome – Zero Unplanned Business Impact

Both parts of the migration were completed five hours before when we guaranteed to the business that services would be restored.

All user, emails, shared mailboxes, Teams data and configuration, SharePoint data and OneDrive data was migrated successfully with minimal issues and zero business impact. A small number of issues were swiftly resolved by IT Naturally’s 24×7 service desk within hours of completing the change.

For our customer this meant that their 24×7 operations were maintained, and their M365 services were all moved before the legacy tenant was switched off with services maintained.

Our partner Binary Tree was impressed at how we maximised the throughput of their tools and said this was the most mailboxes migrated in a single event that they were aware of.