What should IT do with Microsoft 365 in the first 90 days after an acquisition?

Key facts

• Security comes first: confirm who holds Global Administrator rights in the acquired tenant on day one.
• Cross-tenant free/busy and B2B collaboration let people work together without migrating anything.
• Discovery in the first 90 days prevents rushed migrations that break apps and mail flow.
• Do not move domains or mailboxes until the target identity design is decided.
• Communicate early: users mostly care about email address, calendar and file access.

Days 1 to 30: control and security

The acquired company's tenant is now your risk. Before any integration work, confirm control and close obvious gaps.

• List every account with Global Administrator and other privileged Entra ID roles. Remove or reduce where possible and add break-glass accounts you control.
• Confirm MFA coverage for all users, starting with admins. Check whether Security Defaults or Conditional Access is in place.
• Review Microsoft Secure Score and recent sign-in and audit logs for anything unusual.
• Record licensing, renewal dates and who the Microsoft licensing partner or reseller is.
• Identify the acquired company's IT providers, MSPs and any admin access they hold.
• Agree a change freeze on major tenant changes until the integration plan is set.

Days 31 to 60: collaboration without migration

Employees want to schedule meetings and share files right away. Microsoft 365 supports this across tenants without moving data.

Set up an organization relationship in Exchange Online for cross-tenant free/busy. Configure Entra ID cross-tenant access settings to allow B2B collaboration, and consider B2B direct connect for Teams shared channels. Cross-tenant synchronization can create guest or member accounts in each tenant automatically, which makes people searchable in Teams and the global address list.

If the businesses need a shared address book, mail contacts or cross-tenant synchronization can populate it. Keep the setup simple and documented, since it will be partly undone when migration finishes.

Days 61 to 90: discovery and planning

With the business working, run a full discovery of the acquired tenant and decide the end state. Some acquisitions keep two tenants for a long time; most consolidate.

• Inventory mailboxes, shared mailboxes, groups, Teams, SharePoint sites, OneDrive volumes and domains.
• Map identity: on-premises Active Directory, Entra Connect or Cloud Sync, UPN formats and naming clashes.
• List apps using Entra ID sign-in, SMTP relay or service accounts.
• Inventory devices and how they are managed: Intune, Autopilot, Group Policy or none.
• Decide the target identity and domain plan, and whether users keep their current email domain.
• Choose migration tools and draft a wave plan, budget and timeline.

What not to do in the first 90 days

Pressure to show progress after closing leads to rushed moves. These are the most common mistakes.

• Moving mailboxes before deciding the target domain and UPN design, which forces a second change later.
• Removing the acquired company's domain before apps and third-party senders are inventoried.
• Leaving former owners or external providers with admin access.
• Treating Teams and SharePoint as an afterthought, when they usually hold most of the data.
• Skipping user communication, so staff hear about changes from rumors.

What a good 90-day result looks like

At day 90, the acquired tenant is secured and monitored, people can schedule and collaborate across both companies, and leadership has a costed migration plan with a timeline and wave schedule. The actual migration typically starts after that, running 6 to 16 weeks for most SMB and mid-market deals, depending on scope.

How Accred Consulting approaches it

Accred Consulting helps acquirers secure the new tenant, set up cross-tenant collaboration and run discovery inside the first 90 days. The output is a proposal with clear scope, a fixed price and a timeline for consolidation. Our US-based specialists then run the migration in waves with structured check-ins. In one engagement we consolidated about 1,200 users after an acquisition in 8 weeks across 6 waves with no all-hands outage. Handover includes documentation, knowledge transfer and optional support retainers.

Track record: 400+ client projects, 2,000+ migration workstreams and 150,000+ users migrated by a US-based team. Company facts · Case studies

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