Moving Exchange Online mailboxes from one Microsoft 365 tenant to another. It sounds like a copy operation. It is not: a mailbox is an identity, a set of permissions, a calendar with meetings other people organised, delegate rights, forwarding rules, distribution group memberships and an address that the outside world already knows.
Copy the mail and miss the rest, and the estate arrives looking correct and behaving wrongly — shared calendars that will not open, delegates who lost access, meetings that cannot be edited by the person who booked them.
This is the move we have run most often, at every size from twenty-five users to three thousand. For a small estate with clean permissions a migration tool and a competent administrator may be enough — here is how to tell. It is also the one where the schedule is dictated by the platform rather than by effort, which is why we quote a range and explain the arithmetic rather than promising a date.
Read-only. Every mailbox, its size, its archive, and every permission attached to it. Nothing changes. The size distribution comes out of this and sets the schedule for everything that follows.
Users, groups, shared and resource mailboxes built in the destination. Licences assigned. Permissions recreated so mailboxes arrive into containers that are already correct.
Free/busy sharing established between tenants so calendars keep working while the waves run.
Mailboxes move in batches while people keep working in the source. Large archives start first and run in the background. Around twenty mailboxes move concurrently; everything else queues.
Domain moved to the target tenant, MX and autodiscover repointed, addresses set. This is the only part users notice.
Everything that arrived during the window is reconciled. Elevated support for the first working week, when delegate and calendar issues surface if they are going to.
The mailbox size report tells us more than the headcount does. Send what you have and we will tell you the realistic window, including which archives are going to outlast the rest of the project.
From £650 / €750 / $800 · 3–8 weeksNo. Mail moves in the background while people keep working in the source tenant. The only interruption is the cutover window, scheduled for a weekend, and even then mail queues rather than bounces.
If the domain moves with them, yes. A custom domain can only be attached to one tenant at a time, so this is a scheduled cutover event. Where the old brand is being retired, users take the new domain with the old address kept as an alias.
Yes, including delegate rights, Send As and folder permissions — provided they are captured beforehand, which is why the permission map is the first thing we produce. Free/busy works across the tenant boundary during the transition.
Two to three weeks for under 100 users. Six to eight weeks around 500. Two to four months at a thousand or more. Mailbox size distribution matters more than headcount: four users with 2 TB archives can outlast the other 996.
From £650 / €750 / $800, scaling per user. Fixed price, quoted after we have seen the mailbox inventory. Scoping costs nothing.
It queues at the sending server and is delivered once the domain move completes, then a delta pass reconciles anything that landed in the source mailbox during the window. Senders outside your organisation see nothing.
Scoping costs nothing
Answer the form or write to us. You get back a written scope, a sequence and a fixed price — usually within one business day, and nobody rings you unless you ask.