A multi-clinic group migrates 400 mailboxes with zero downtime

Eleven sites, 400 staff and a legacy mail platform at end of life. Clinics run appointment books, so the migration window was not “out of hours”, it was “between the last patient and the first one tomorrow”.

There was also no backup of the existing mail data at all, which nobody had realised until we asked.

Assess, plan, migrate, optimise. We mapped every mailbox, shared calendar and clinical integration first, then migrated in eleven site-by-site waves, each with a rollback position.

Backup was stood up before anything moved, not after. Conditional access and enforced MFA went in as part of the migration rather than as a later project, because doing it twice costs more.

400 mailboxes migrated with zero downtime during clinic hours and no clinical session affected.

The group now runs a hardened tenant with backup, MFA and activity monitoring, and its licensing was reduced after the review found seats nobody was using.