Mailbox migrations rarely happen overnight. For weeks, half the company sits on Exchange Server and half in Microsoft 365, and people on the two sides often cannot see each other’s calendars properly. That is where meetings get missed and the migration project gets the blame. Here is how to run an Exchange to Microsoft 365 migration without calendar gaps.

What goes wrong in the middle

What the sync does, and what it does not

Be clear about the split, because migrations punish vague promises. AddSync keeps calendars, contacts and tasks in step between Exchange Server and Microsoft 365, in the direction you choose, while people move over in waves. It does not move mailboxes. Use Microsoft’s tools or your migration partner for that. The sync covers the gap they leave: the weeks when two systems must behave like one.

A plan that survives the users

1. InventoryList who moves in which wave and who shares calendars across waves.
2. Pilot groupSync a small group first. Check events, changes and cancellations.
3. Run the wavesKeep the sync on while people move. The calendars stay trustworthy.
4. Switch offWhen the last mailbox has moved, end the sync and keep working in Microsoft 365.

Example: three waves

Example, not a real customer. The numbers are yours to change. The idea is to start small and keep the sync on until the end.

Common mistakes

• Planning waves without listing who shares calendars across them.
• Switching the sync off before the last mailbox has moved.
• Expecting a sync to move mailboxes.
• Leaving AI and calendars out of the migration plan until the end.

What about Exchange hybrid?

A hybrid deployment can share free and busy information between the two sides, and that covers part of the problem. If you also need full event details, contacts and tasks in both places, check what your hybrid setup really provides before you rely on it. A short test with a few users in different waves answers it quickly.

And the AI question

Your people want AI on mail now, not after the migration. AddSync AI works on Exchange Server today and on Microsoft 365 after the move, with the same filter, approval and journal on both sides. Microsoft Copilot does not reach on-premises mailboxes, so waiting staff get nothing in the meantime. See AddSync AI vs Microsoft Copilot.

Questions and answers

Does AddSync migrate mailboxes?

No. It keeps calendars, contacts and tasks in step. Use Microsoft’s migration tools or a partner for the mailbox move.

Can I choose which way data flows?

Yes. You choose the direction of the sync.

What happens when the migration is over?

You end the sync. Microsoft 365 stays as your system.

Is it priced per account?

Yes. See the pricing page.

Written by the AddSync team.