Microsoft 365 & Copilot in the Enterprise · Chapter 3 of 12
The workloads, and what actually breaks
Exchange, SharePoint, OneDrive and Teams — what each replaces, and the specific failure that catches each migration.
Four services carry almost all the value, and each has one characteristic way of going wrong in migration. Generic advice (‘test first’) is useless here; the specific failures are what you need.
Exchange Online — email
Replaces on-premises Exchange Server. Note that Microsoft frames hybrid as a supported permanent end state, not merely a phase you pass through — plenty of large organisations run both indefinitely and always intended to.
| Migration type | Source | When to use it |
|---|---|---|
| Cutover | Legacy Exchange 2003/2007 | Everything at once. Hard cap 2,000 mailboxes, realistically 150. For modern sources Microsoft now steers you to Hybrid Express instead. |
| Staged | Exchange 2003/2007 only | Batched. Both eligible source versions are long out of support, so this path is legacy-only in practice. |
| Hybrid — Full | Exchange 2010+ | Many thousands of mailboxes, ongoing shared calendar availability. Indefinite coexistence supported. |
| Hybrid — Minimal | Exchange 2010+ | Hundreds to low thousands. Directory sync, no shared free/busy. |
| Hybrid — Express | Exchange 2010+ | Small organisations, about two weeks. Now the recommended path over Cutover. |
| IMAP | Non-Exchange systems | Email only. Calendars, contacts and tasks do not come across. |
The characteristic failure: shared calendar availability breaks after cutover. Users can no longer see whether colleagues are free. The cause is almost always a stale or blank sharing endpoint in the Organization Relationship, or a missing Autodiscover mapping — Autodiscover being the mechanism by which Outlook works out which server a mailbox lives on. It looks like a mail problem and is a configuration problem.
A retirement to check for right now. Exchange Web Services (EWS) — the older programming interface for talking to Exchange — is deprecated. Phased disablement begins 1 October 2026 and permanent retirement is 1 April 2027.
Anything built against it must move to Microsoft Graph, the unified modern API across all of Microsoft 365.
This bites in a predictable place: CRM connectors, meeting-room booking systems, archiving tools and bespoke mail automation. They are rarely in anyone's application inventory, and they all use EWS.
SharePoint Online — documents
Replaces on-premises SharePoint Server and, more often, ordinary network file shares.
The characteristic failure is a security regression, not an inconvenience
Windows file shares use NTFS permissions, which include an explicit deny — a rule saying this person must not see this, overriding any allow they inherit.
SharePoint has no equivalent. Microsoft's own migration guide states it plainly: after migration, the most restrictive permission is mapped across, all advanced NTFS permissions are removed, and for explicit deny permissions the content may become accessible.
So content that was deliberately blocked can quietly become readable. Nothing errors. Nobody is notified.
Audit your deny rules before you migrate, not after. And hold this thought — it is the same mechanism that becomes the Copilot oversharing problem in chapter 8.
The same guide flags two mundane blockers worth clearing early: path-length limits, and filenames containing # or %, which are disabled by default.
OneDrive — personal files
Replaces the mapped home drive — Microsoft's own guide calls it a ‘Z drive’ on networked computers. The mechanism that moves people across is Known Folder Move, which redirects Desktop, Documents and Pictures into OneDrive silently.
The characteristic failure: an existing Group Policy already redirecting those folders somewhere else. The two mechanisms fight, and the result is either a silent no-op or a confusing half-migrated state. Check for folder-redirection policy before enabling Known Folder Move, and stage the rollout — the commonly cited throttle is around a thousand devices a day, which is about controlling support-ticket volume rather than any technical limit.
Teams — chat, meetings and calls
Replaces Skype for Business, which was retired on 31 July 2021, and increasingly the office phone system.
The characteristic failure follows from a structural fact: Teams is not one service. Chat and calendar live in Exchange. Files live in SharePoint. Identity is Entra ID. Teams is a front end over three other systems — so Teams migrations fail in ways that present as Exchange or SharePoint problems, and get diagnosed slowly for that reason.
A concrete example of the seam: each private channel gets its own SharePoint site — but only after a user manually opens the Files tab once, not at channel creation. Any script that tries to pre-provision them in bulk simply finds nothing there.
Teams Phone — four ways to reach the public phone network
| Model | What you need | Who owns the phone service |
|---|---|---|
| Calling Plan | Teams Phone licence plus a calling plan from Microsoft | Microsoft entirely, with a 99.999% reliability commitment |
| Operator Connect | A contract with a certified operator, enabled in the Teams admin centre | Shared — the operator carries calls, Microsoft runs Teams |
| Teams Phone Mobile | A certified mobile operator; the user's own SIM number becomes their Teams number | The mobile operator. Does not support dial-in conferencing or auto-attendant numbers — personal numbers only |
| Direct Routing | Your own certified session border controller | You and your chosen carrier. Most flexible, most work |
All four can coexist in one tenant, assigned by persona. Which you pick is usually decided by your existing carrier contracts rather than by technology — and, as chapter 12 explains, it is frequently the longest single dependency in the whole programme.
Check yourself