Contents
What sets the timeline
Three things decide how long a migration takes: how many people move, how much mail and file data comes with them, and how ready your user accounts are. A 20-person office with small mailboxes is a different job from a 300-person company with ten years of archived mail and shared drives on a local server.
| Company size | Typical duration | What takes the time |
|---|---|---|
| Up to 25 users | 1 to 2 weeks | Licensing, DNS changes and a small amount of mail. Most of the work fits around normal office hours. |
| 25 to 100 users | 2 to 4 weeks | Mailbox sizes, shared mailboxes and calendars, and getting staff comfortable with Teams. |
| 100 to 500 users | 4 to 8 weeks | Batch planning by department, identity setup across branches, and moving large file shares to SharePoint. |
These ranges assume the current mail system is reachable and an administrator can grant access to it. Old servers, mail kept in local PST files or archives nobody has licenses for all add time, which is why the first step is an inventory.
A typical migration, step by step
Most migrations follow the same six steps. Smaller companies move through them in days; larger ones repeat the middle steps in batches.
- Inventory users, mailboxes and shared data List every user, shared mailbox, distribution list and file share, with their sizes. This shows how much data has to move and what could be archived instead.
- Choose licenses and set up the tenant Pick a plan for each role, for example Business Standard for office staff and Business Basic for frontline users. Then create the Microsoft 365 tenant and verify your domain.
- Connect Microsoft Entra ID and test sign-in Create or sync user accounts in Microsoft Entra ID, turn on multi-factor authentication and test sign-in with a pilot group before any mail moves.
- Migrate mailboxes in batches Copy mail, calendars and contacts in batches, usually department by department. Older mail copies in the background while people keep working in the current system.
- Cut over DNS and mail flow Point the MX and autodiscover records at Microsoft 365, ideally on a Friday evening. New mail starts arriving in the new mailboxes within hours, and a final sync picks up anything sent during the switch.
- Move files to SharePoint and roll out Teams Move shared drives into SharePoint and Teams channels, set permissions to match how departments work, and show staff where their files now live in Teams and OneDrive.
A pilot group of five to ten people goes through every step first. Problems show up with a handful of users instead of the whole company.
How to avoid downtime
Done well, nobody loses email at any point. The risk sits in three places: sign-in, the DNS switch, and laptops or phones still pointing at the old server.
Lower the DNS record lifetimes a few days before cutover, schedule the switch outside business hours, and keep the old server running for two weeks as a fallback.
After the move
The first two weeks bring the most questions: Outlook profiles, shared calendars, phones that need the account added again. Plan for an engineer to be on call during that period.
After that, the work becomes ordinary administration: new starters and leavers, license changes, security alerts and backups. Microsoft keeps the service running, but backing up your Microsoft 365 data is still your responsibility.
Frequently asked
Will staff lose email during the move?
No. Mail is copied while the old system keeps working, and the switch happens in one evening. At most, new messages take a few hours to start arriving in the new mailbox.
Do we need new licenses before we start?
You need licenses for each person before their mailbox moves, but not all at once. Most companies buy a few for the pilot group, then the rest before the main batches.
Can old mail be archived instead of migrated?
Yes. Mail older than a chosen date can go to an archive mailbox or into a read-only export. For companies with large, old mailboxes this shortens the migration considerably.