The seven steps, and the decision that gates each one
A tenant migration fails in planning long before it fails on a weekend. EPC Group therefore treats the plan as a chain of gates: no step opens until the previous one has produced a written decision. The sequence below is the one EPC Group uses whether the source is an on-premises Exchange organization, a second Microsoft 365 tenant acquired in a merger, or a Google Workspace domain. The tooling differs by source; the gates do not.
- Choose the migration path. Microsoft documents cutover, staged and hybrid moves from Exchange Server, IMAP moves from any IMAP-capable system, a dedicated Google Workspace path, and cross-tenant moves between Exchange Online organizations. If the server is older than Exchange 2003, IMAP is the only option, and staged and hybrid both require directory synchronization (Microsoft Learn, Decide on a migration path in Exchange Online, accessed 2026-10-20). Gate: a signed path decision.
- Inventory the source. Mailboxes, holds, delegates, shared mailboxes, groups, public folders, SharePoint and OneDrive content, and the applications that send mail through the tenant. Gate: an inventory with no unknown owners.
- Settle identity. Directory synchronization, matching attributes for the target objects, and the sign-in experience users will see. Gate: a pilot user can sign in to the target with the intended credential.
- Run the pilot. A deliberately mixed group, with exit criteria written before it starts. Gate: all exit criteria met or the plan revised.
- Size and schedule waves. Grouped by dependency, bounded by Microsoft's batch limits and by measured throughput. Gate: a wave calendar the business has approved.
- Execute the cutover weekend. Final sync, MX change, verification, licensing. Gate: the verification checklist is complete before the batch is deleted.
- Decommission. Remove the source after a monitoring period, with endpoints and organization relationships cleaned up. Gate: no mail or file traffic reaches the source for an agreed interval.
Inventory: what the source tenant must tell you before anything moves
The inventory step is where most surprises are found, so EPC Group runs it as a data exercise rather than a questionnaire. Three categories need particular attention. First, holds and compliance state: Microsoft states that mailboxes on any type of hold are not migrated in a cross-tenant move and the move is blocked, and that only user-visible content such as email, contacts, calendar, tasks and notes moves (Microsoft Learn, Cross-tenant mailbox migration, accessed 2026-10-20). Legal hold therefore has to be resolved with counsel before a wave is scheduled. Second, permissions: Microsoft notes that explicitly applied mailbox permissions such as Send As, Full Access and Send on Behalf migrate in a hybrid move, while inherited permissions and permissions granted to objects that are not mail-enabled do not, so they should be made explicit before migration (Microsoft Learn, Exchange Server hybrid deployments, accessed 2026-10-20). Third, licensing: a cross-tenant move requires a per-user Cross Tenant User Data Migration license, and Microsoft states that migrations fail without it and that no exceptions are offered (Microsoft Learn, Cross-tenant mailbox migration, accessed 2026-10-20).
Choosing the pilot group
The temptation is to pilot with the IT department alone. EPC Group resists that, because IT users rarely exercise delegated mailboxes, room bookings or public folders the way the business does. The pilot group EPC Group assembles has four properties: it contains people who delegate to each other, so permission carry-over is tested; it contains at least one heavy mobile user, because Microsoft notes that some older Exchange ActiveSync devices might not reconfigure automatically after a move (Microsoft Learn, Exchange Server hybrid deployments, accessed 2026-10-20); it contains a user from a department with few external dependencies, so a rollback is low-cost; and it contains the change-management lead, who will write the communications for later waves from first- hand experience. Microsoft's own guidance frames the pilot as a small set of users used to validate process, performance and user communication before the remainder of the schedule is set (Microsoft Learn, Guide to migrating file shares to OneDrive, Teams, and SharePoint, accessed 2026-10-20).
Sizing the waves
Wave size is bounded from above by Microsoft and from below by the business. For cross-tenant Exchange Online moves, Microsoft advises not exceeding 2,000 mailboxes per batch and recommends submitting batches two weeks before the cutover date because the synchronization phase has no impact on end users (Microsoft Learn, Cross-tenant mailbox migration, accessed 2026-10-20). For a cutover migration from on-premises Exchange, the ceiling is 2,000 mailboxes, but Microsoft describes 150 or fewer as the more reasonable number given the time taken to create and migrate users (Microsoft Learn, Ways to migrate multiple email accounts to Microsoft 365 or Office 365, accessed 2026-10-20). For hybrid moves, Microsoft advises determining average mailbox size and connection throughput, then calculating expected transfer speed and planning moves accordingly (Microsoft Learn, Exchange Server hybrid deployments, accessed 2026-10-20). Within those bounds, EPC Group groups users by shared mailbox and delegate relationships so that a team moves together, and keeps a wave small enough that the help desk can absorb its first-morning calls.
Shared mailboxes, groups and public folders
These three object types are where tenant migrations diverge from simple mailbox copies. Shared mailboxes move as mailboxes, and in a cross-tenant move Microsoft states that mailbox permissions stored in the mailbox move when both principal and delegate are moved, while Send on Behalf must be re-stamped on the target after the move (Microsoft Learn, Cross-tenant mailbox migration, accessed 2026-10-20). Microsoft 365 Groups are not supported by the cross-tenant mailbox migration feature (same source), so Teams, Planner and group-connected SharePoint sites need their own migration track, which is why EPC Group's M&A tenant migration practice plans collaboration content separately from mail. Public folders use batch migration from Exchange Server 2010 SP3 RU8 or later (Microsoft Learn, Public folders in Microsoft 365, Office 365, and Exchange Online, accessed 2026-10-20); Microsoft advises completing user migration first, allows only a single public folder migration batch, and asks that folder names containing a backslash or forward slash be renamed before migration (Microsoft Learn, Use batch migration to migrate Exchange Server public folders to Exchange Online, accessed 2026-10-20).
The cutover-weekend runbook
EPC Group's runbook is a timed checklist, not a narrative. In the week before, the MX record's time-to-live is lowered; Microsoft recommends an interval of 3,600 seconds or less before starting so that sending systems refresh the location of the mail system quickly (Microsoft Learn, Migrate other types of IMAP mailboxes to Microsoft 365 or Office 365, accessed 2026-10-20). On the weekend, the final synchronization completes, the MX record is pointed at Microsoft 365, and the team confirms that mail is being delivered to the new mailboxes. Microsoft states that it can take up to 72 hours for the email systems of customers and partners to recognize the changed MX record, that the batch should not be deleted before then, and that the batch's Last Synced Time must be more recent than the moment mail began routing directly to the new mailboxes (Microsoft Learn, Migrate email to Exchange Online using the Exchange cutover method, accessed 2026-10-20). Licenses are assigned the same weekend; Microsoft notes that an unlicensed migrated mailbox is disabled when the 30-day grace period ends (same source). Autodiscover DNS, the welcome message and a staffed help desk on Monday morning close the runbook.
Documented work
ANON-CASES: EPC Group engagement data for tenant migration covers mid-size commercial organizations consolidating or relocating Microsoft 365 tenants, delivered by the firm's Microsoft 365 consulting practice. Client names are not published here. Engagements are described by sector and organization size only, and no outcome figures are quoted that cannot be traced to a signed deliverable. EPC Group is a Microsoft Solutions Partner with all six Solutions Partner Designations as of October 2026.
Primary sources
- Cross-tenant mailbox migration — Microsoft Learn, accessed 2026-10-20
- Ways to migrate multiple email accounts to Microsoft 365 or Office 365 — Microsoft Learn, accessed 2026-10-20
- Exchange Server hybrid deployments — Microsoft Learn, accessed 2026-10-20
- Use batch migration to migrate Exchange Server public folders to Exchange Online — Microsoft Learn, accessed 2026-10-20
- Migrate email to Exchange Online using the Exchange cutover method — Microsoft Learn, accessed 2026-10-20
- Decide on a migration path in Exchange Online — Microsoft Learn, accessed 2026-10-20
- Migrate other types of IMAP mailboxes to Microsoft 365 or Office 365 — Microsoft Learn, accessed 2026-10-20
Frequently Asked Questions
How is the source tenant inventoried?
EPC Group exports every mailbox with its size, archive, holds, delegates and last-logon date, then lists shared mailboxes, distribution groups, Microsoft 365 Groups, public folders and the SharePoint and OneDrive volumes. Holds matter early: Microsoft states that mailboxes on any type of hold are not migrated cross-tenant and the move is blocked (Microsoft Learn, Cross-tenant mailbox migration, accessed 2026-10-20). The inventory also records Send As and Full Access grants so they can be recreated where the tooling does not carry them.
How is the pilot group chosen?
The pilot group is chosen to exercise every workload the production waves will touch, not to be the easiest group to move. EPC Group selects users who share delegated mailboxes with each other, at least one mobile-heavy user, one person who relies on public folders, and members of the IT and change teams. Microsoft frames the pilot as a small set of users used to validate the process, performance and user communication before scheduling the remainder (Microsoft Learn, Guide to migrating file shares to OneDrive, Teams, and SharePoint, accessed 2026-10-20).
How are waves sized?
Waves are sized by dependency first and volume second. For cross-tenant Exchange Online moves Microsoft says not to exceed 2,000 mailboxes per batch and recommends submitting batches two weeks before the cutover date because synchronization has no end-user impact (Microsoft Learn, Cross-tenant mailbox migration, accessed 2026-10-20). For a cutover migration from on-premises Exchange the cap is 2,000 mailboxes, with 150 or fewer described as the more reasonable number (Microsoft Learn, Ways to migrate multiple email accounts to Microsoft 365 or Office 365, accessed 2026-10-20).
What happens to shared mailboxes, groups and public folders?
Shared mailboxes move as mailboxes, and explicitly granted Full Access, Send As and Send on Behalf permissions migrate in a hybrid move while inherited permissions do not (Microsoft Learn, Exchange Server hybrid deployments, accessed 2026-10-20). Cross-tenant moves do not support Microsoft 365 Groups, so Teams and group-connected sites need a separate plan (Microsoft Learn, Cross-tenant mailbox migration, accessed 2026-10-20). Public folders move last, in a single batch, after user mailboxes are complete (Microsoft Learn, Use batch migration to migrate Exchange Server public folders to Exchange Online, accessed 2026-10-20).
What is the cutover-weekend runbook?
The runbook lowers the MX record TTL in advance, completes the final sync, repoints MX, verifies that the migration batch synced at least once after mail began routing to the new tenant, and only then deletes the batch. Microsoft says it can take up to 72 hours for other mail systems to recognize a changed MX record and to wait at least that long before deleting the batch (Microsoft Learn, Migrate email to Exchange Online using the Exchange cutover method, accessed 2026-10-20). Licenses, Autodiscover DNS and the user welcome message close the weekend.
Talk to a senior architect
If your organization is planning a tenant migration, a 30-minute discovery call with a senior EPC Group architect will cover the path decision, the inventory approach and a first-pass wave calendar. See the Microsoft 365 consulting service hub for engagement structures.
Email contact@epcgroup.net or call 888-381-9725.
a Microsoft Gold Partner (2000 until Microsoft retired the program in 2022) — today holding all six Microsoft Solutions Partner Designations.
Documented engagements
Each record publishes a redacted primary-source document — a client purchase order, executed agreement or payment record — with a SHA-256 hash you can recompute, and states what the document does not establish.
- Tarleton State University — Contracted — countersignedCase study: Tarleton State: Office 365 Cross-Tenant Migration (2021)