Folding acquired firms into one tenant, several every month

A professional services group was growing by acquiring regional firms, often several in a single month. Each firm arrived with its own Microsoft 365 tenant, its own domains and DNS host, and often its own on-premises directory. The group wanted every firm on one tenant and one standard technology stack, and it wanted new firms onboarded without disrupting client work.

• 15+Acquired firms onboarded
• SeveralFirms in flight each month
• 45+Domains tracked and moved
• 1Group tenant at the end

The situation

The group was rolling out a standard stack to every firm it bought: network, cloud servers, devices and phones, delivered by its IT integration partner. Accred handled the Microsoft 365 and identity side of each firm's onboarding, working alongside that partner so the mailbox move lined up with everything else happening at the site.

No two firms were set up the same way. Some had a tidy tenant and one domain. Others had several domains spread across registrars, years of shared mailboxes and permissions nobody had written down, and a local Active Directory still holding the real user records.

What made it hard

• Access first: registrar, DNS and admin access often sat with a former owner or an outside provider and had to be tracked down per firm
• Many domains per firm, each needing DNS moves plus SPF, DKIM and mail-filtering changes in the right order
• Hidden mailbox structure: shared, resource and delegated mailboxes, calendar permissions and distribution lists that had to arrive intact
• Old addresses still in use: replies to old threads and existing meeting invites had to keep resolving after the move
• Overlap: several firms at different stages at once, all during busy, deadline-driven client work

How we approached it

1. One master workbook per firm

Discovery for each firm exported users, groups, shared mailboxes, mailbox and calendar permissions, admin roles and mailbox sizes into a single master workbook. That workbook became the source of truth for mapping, scripting and validation, and the pattern was the same for every firm.

2. Domains and access tracked like a project of their own

A group-wide tracker recorded every firm's domains, registrar, name servers, DNS and admin access, and whether each domain and DNS zone had moved. SPF and DKIM were checked per domain before cutover. When access was missing, it showed up weeks ahead of time rather than on cutover night.

3. Identity cleaned up before mail moved

User attributes were corrected in the directory and synced, so people arrived in the group tenant with the right names, titles and addresses. Legacy X500 addresses were carried over so replies to old messages and existing meeting invites still reached the right person, and meeting links were checked after cutover.

4. Scripted moves for everything that isn't a user mailbox

User mailboxes moved with Quest migration tooling. Distribution lists, shared mailboxes, resource mailboxes and proxy addresses moved with a reusable PowerShell kit that exported from the source and rebuilt in the target, so the hundredth shared mailbox was handled exactly like the first.

5. A review pass after every firm

After each cutover we compared the result against the master workbook, picked up mailboxes that had been added late, reapplied calendar permissions and closed out access. Fixes went back into the kit for the next firm.

Results

• 15+ acquired firms brought into the group's single Microsoft 365 tenant
• Several firms onboarded in parallel each month without one blocking another
• Staff kept their addresses, mail history, shared mailboxes and calendars
• 45+ domains moved and tracked with a clear record of DNS and access for each
• The group kept a per-firm workbook template and script kit for future acquisitions

What we'd tell a peer in the same spot

With serial acquisitions, access and domains are the long pole, not the mailbox copy. Start tracking registrar and DNS access the day the deal closes, script everything that isn't a user mailbox, and treat every firm as the same project with different data.