What actually moves when you move to Microsoft 365

Ask three IT companies how long a Microsoft 365 migration takes and you will get three numbers and no working. Microsoft publishes something better and almost nobody reads it: a set of duration tables built from analysis of previous customer migrations, split by mailbox size. Onboarding from an on-premises Exchange server, mailboxes between 10 and 50 GB, the median is two days and the ninetieth percentile is six.

That is one number out of a page of them, and the page is free. Most of the decisions in a migration are like that. They are already written down, they are boring, and getting one of them wrong is what turns a weekend job into a fortnight of people asking where their calendar went.

Four ways in, and only two of them are yours

Microsoft supports four routes onto Exchange Online, and most sales conversations blur them into one. Cutover, staged, hybrid, IMAP. There is a fifth if you count importing PST files, which is less a migration than a rescue operation.

A cutover moves everything at once. It works from Exchange 2003 onwards, and it takes mail contacts and mail-enabled groups with their membership across as well as the mailboxes. The documented ceiling is 2,000 mailboxes, at which point Microsoft immediately talks itself down: it is more reasonable to migrate 150 users or less, because performance suffers above that. If you have fourteen people, none of this constrains you.

A staged migration exists for Exchange 2003 and 2007 above 2,000 mailboxes. Hybrid is for Exchange 2013 or later, or for anyone who genuinely needs both worlds running side by side for months. Both require directory synchronisation to be set up first, which is a project in its own right. Neither is a sensible answer for an office of thirty people who want their email somewhere else by Christmas.

IMAP covers everything the other three do not: a web host's mail server, a cPanel box, an appliance in the corner that nobody has the login for. Microsoft is direct about the bottom end of the range too. If your mail server is older than Exchange 2003, IMAP is your only option.

So a small business really has two choices rather than four. Cutover if there is an Exchange server. IMAP, or the Google Workspace route, if there is not.

What IMAP leaves behind

Read this part twice if your mail currently sits with a hosting company.

An IMAP migration moves items in the inbox and other mail folders. It does not move contacts. It does not move calendar items. It does not move tasks. Microsoft says so in one flat sentence in its own documentation, and it is the most expensive surprise in this entire subject, because it surfaces on the Monday after everyone has been told the move went perfectly.

Four more limits belong on the plan before anybody agrees a date.

• IMAP does not create mailboxes. Every user and every licence has to exist in the tenant first, which means the licence bill starts before the migration does.
• Five hundred thousand items per mailbox, migrated newest to oldest. A long-serving mailbox can hit that.
• Thirty-five megabytes is the largest message that will move. Anything bigger stays where it is.
• You need administrator credentials on the source, or every user's password. If you have neither, you reset the passwords, and then nobody can get back into their old mailbox unless you hand the new ones out afterwards.

There is one more trap, and it generates a phone call rather than a ticket. Microsoft strongly recommends turning off retention and archival policies before any migration, because the migration tooling has no idea they exist. Anything a policy moves or deletes while the copy is running gets flagged as missing. Their own phrasing is worth borrowing: perceived data loss rather than actual data loss, which makes it much harder to identify actual data loss. You spend the afternoon auditing a list of failures that are not failures, and a genuine one can sit in the middle of it unnoticed.

How long it takes, from Microsoft's own numbers

These come from a page called migration performance and best practices, and Microsoft describes them as guidelines based on a data analysis of previous customer migrations. P50 is the median. P90 is the number your plan should actually respect.

Above 200 GB in a single mailbox, every route in the table reads "not supported". Find out now whether anybody in your business is near that, because the answer changes the shape of the project rather than its timing.

Two things follow from the spread. A plan built on the median has no slack in it at all, and the copy itself is not an outage, because it runs in the background while everybody carries on using the old system. There is also a factor outside your control: Microsoft treats mailbox moves as a discretionary workload and throttles them to protect the service for everyone else. Your migration can be slowed down by other people's Tuesday.

The 50 GB wall

Microsoft 365 Business Basic, Business Standard and Business Premium all give a user a 50 GB mailbox. Enterprise E3 and E5 give 100 GB. That is the whole story on the primary mailbox, and it does not vary by how much you paid inside the Business range.

The wall arrives in three steps rather than one. A warning at 49 GB, prohibit send at 49.5 GB, prohibit send and receive at 50 GB. People notice the middle one, usually while trying to reply to a client.

Archive storage is where the tiers genuinely differ, and it is the number that should drive the licence choice for anybody with a long history. Business Basic and Business Standard get a 50 GB archive. Business Premium gets auto-expanding archiving: each user starts with 100 GB and more is added as it fills, up to 1.5 TB. So a partner with fifteen years of mail and a pile of PST files on their desktop is a Premium decision, and it is better made before the migration than during it. One piece of small print that catches people out: a retention policy only moves messages into the archive automatically if the primary mailbox is larger than 10 MB.

The rest of the licence question, the security and device-management side, we covered separately in Business Standard against Business Premium.

Shared mailboxes, and the rule that costs people money

A shared mailbox does not need its own licence. On a business running info@, accounts@, bookings@ and sales@, that is a real saving and plenty of people are paying for it anyway.

Two conditions hang off it. An unlicensed shared mailbox is capped at 50 GB, and lifting that to 100 GB means putting an Exchange Online Plan 2 licence on the mailbox itself, or a Plan 1 with an archiving add-on. Separately, anybody who opens that shared mailbox needs an Exchange Online licence of their own. Meeting rooms and equipment mailboxes follow the same pattern and hit the same ceiling.

We find both mistakes in the same building. Every shared address licensed as though it were a person, and the accounts mailbox sitting at 49.6 GB, quietly unable to send anything.

The server you are leaving behind

Extended support for Exchange Server 2016 and Exchange Server 2019 both ran out in October 2025. Microsoft's lifecycle pages put 2016 on sale in October 2015 with mainstream support ending in October 2020, and 2019 on sale in October 2018 with mainstream ending in January 2024. Different products, five years apart, and they stopped being patched at the same moment.

The box keeps working, which is exactly the problem. It is the same argument as Windows 10 reaching end of support, with one difference that matters: a mail server is reachable from the internet by design. Anything found in it now stays found.

Do not switch it off the day the migration finishes, though. Keep it until the new mail flow is verified and you have run at least one month end through it. Then retire it properly, with the data accounted for, rather than leaving a powered-down server under a desk as somebody's idea of a backup.

The Monday morning list

Copying the mail is the easy half. Four things sit around it, and every one of them has delayed a migration we have been involved in.

The domain comes first, and it is not a technical job. Before a cutover can run, your primary domain has to be verified as a domain you own inside the Microsoft 365 tenant. That means access to the registrar, and the registrar login usually belongs to whoever set the domain up in 2011 and left in 2019. Start this the day the project is agreed. It is routinely the longest lead time in the whole exercise and none of it involves email at all.

Licences come next, and the documentation is blunt about it. After a cutover, every on-premises mailbox user exists as a new user in the tenant, but you still have to assign licences. An unlicensed user is a user who does not receive mail.

Then the MX record, which is what actually moves your mail. Until it changes, mail sent to a migrated user is still routed to their old mailbox on the old server. And anything that arrives at the source after the copy has finished is not migrated, so the copy and the switch need to sit close together with a final sync between them. The Autodiscover DNS record is a post-migration job, and it is the thing that stops Outlook asking everyone in the building for a password on the same morning.

Last, the desktops. Microsoft's own wording is that administrators or users must configure desktop computers. Translated, that is a new Outlook profile per person, plus phones, plus whoever has a scanner or a line-of-business system sending mail through the old server. That is the hour per head that nobody puts in the quote.

What it costs, and what we will not promise

Microsoft 365 migration is £77 per mailbox excluding VAT, published on our pricing page rather than quoted on request, and it covers the full mailbox and data migration with a managed user cutover. Licences are supplied at list price and managed for you. We do not mark them up, because we would rather be paid for the work than for standing between you and Microsoft. On-site project work such as a network or server install is £855 a day. If a server stays, management is £162 per server per month. Afterwards the day-to-day sits inside the managed plans at £50, £77 and £108 per user per month excluding VAT, with out-of-scope work at £86 an hour for clients on a plan and £126 without one.

Here is what we will not put in a quote. Every migration has a window in it somewhere. Mail migrations are staged so your staff keep working through them, and the final cutover runs outside business hours, but a server move sometimes needs a short planned window, and we agree that with you in advance rather than discovering it together on the night. Anyone promising a move of this size with no interruption at all is either doing something clever they have not explained, or has not done many.

We are also not Microsoft. When a tenant does something inexplicable at two in the morning we open the case and chase it, and the fix is still theirs to ship.

One more line on the invoice that is worth expecting. Microsoft keeps the service running; your content is a separate question, and retention policies and recycle bins are not a backup you can restore from a point in time. What we recommend instead is on the backup and recovery page, and it is its own product with its own cost.

Before you book a date

Six questions. None of them needs an engineer, and the answers decide the price and the timeline between them.

1. How big is the biggest mailbox? Not the average. The biggest, because that one sets the schedule and it is the one that might not fit.
2. How many shared and resource mailboxes are there? Count the addresses nobody thinks of as people: info@, the room calendars, the alias that only the printer uses.
3. Where do the contacts and calendars actually live? On the server, or in a file on somebody's laptop. This decides whether IMAP is survivable.
4. Who can log into the domain registrar? If the honest answer is nobody, that is the first job and it can take weeks.
5. What else sends email through the old server? Scanners, alarm panels, the accounts package, a website contact form. They will all stop, and they will stop quietly.
6. What has to survive the move for legal or contractual reasons? That is your call and your obligation. We build the retention to match it; we do not decide what it should be.

If you would rather hand the counting to somebody else, we will do that part before anyone signs anything. Ring 020 3411 1886 or use the contact form.