Planning an Exchange to Microsoft 365 Migration Step by Step

Most migration problems are planning problems. The mailbox that fails to move on cutover night was oversized three weeks earlier.

This post walks through the planning work for moving an on-premises Exchange organization to Exchange Online: what to discover, how to size the job, how to pick a method, and how to build a timeline you can actually keep.

Discovery: know what you are moving

Start with the Exchange organization itself. You need the server versions and cumulative update levels, the number of mailboxes by type, total data size, the largest mailboxes, and everything that is not a user mailbox: shared mailboxes, resource mailboxes, public folders, distribution groups, mail contacts, and journaling or transport rules.

Pull this from the Exchange Management Shell rather than from memory. A few cmdlets give you most of what you need.

Get-ExchangeServer | Select Name, Edition, AdminDisplayVersion
Get-Mailbox -ResultSize Unlimited | Group-Object RecipientTypeDetails | Select Name, Count
Get-Mailbox -ResultSize Unlimited | Get-MailboxStatistics | Sort TotalItemSize -Descending | Select DisplayName, ItemCount, TotalItemSize -First 25
Get-PublicFolderStatistics -ResultSize Unlimited | Measure-Object TotalItemSize -Sum
Get-TransportRule | Select Name, State, Priority
Get-ReceiveConnector | Select Name, Bindings, RemoteIPRanges

Discovery outside of Exchange

The second half of discovery is everything that talks to Exchange without being a mailbox. This is where cutovers go wrong, because nobody owns these items and nobody remembers them until they break.

Walk the receive connector logs for a couple of weeks. Every IP that submits mail anonymously is a device or an application you will need to repoint. Check the line-of-business apps that send notifications, the copiers that scan to email, the backup software that emails reports, the monitoring system, the website contact form, and the accounting package.

• SMTP relay clients: printers, scanners, alarm panels, UPS cards, ERP and CRM systems.
• Applications that read mailboxes with EWS, MAPI or IMAP, such as ticketing systems and fax gateways.
• Mobile devices using ActiveSync and how they authenticate today.
• Third-party archiving, journaling, signature or spam filtering products in the mail path.
• DNS ownership: who can change MX, Autodiscover and SPF records, and how fast.
• Identity: is Active Directory synced to Entra ID already, and are UPNs routable?

Get-ReceiveConnector "SERVER\Default Frontend SERVER" | Get-MessageTrackingLog -EventId RECEIVE -Start (Get-Date).AddDays(-14) -ResultSize Unlimited | Where-Object { $_.Source -eq "SMTP" } | Group-Object ClientIp | Sort Count -Descending | Select Name, Count

Sizing: data, bandwidth and quotas

Total mailbox size sets the length of the data copy. The number that matters is not the sum, it is the sum divided by what your internet uplink and the source server can push in a night. A 400 GB organization on a 100 Mbps uplink is a very different job from the same data on a gigabit fiber circuit. Measure a real move of one mid-sized mailbox in the pilot and extrapolate from that.

Check every large mailbox against the target quota before you move it. Exchange Online gives 50 GB primary mailboxes on Business plans and 100 GB on E3 and E5. A 70 GB mailbox on a Business Standard license will fail at the end of the copy, after you have already spent the bandwidth. Plan an archive, a cleanup, or a different license for those users ahead of time.

Items over 150 MB cannot be moved into Exchange Online at all. Find them now so they are not a surprise in the move report.

Get-Mailbox -ResultSize Unlimited | Get-MailboxStatistics | Where-Object { $_.TotalItemSize.Value.ToGB() -gt 45 } | Select DisplayName, TotalItemSize
Get-Mailbox -Identity user@domain.com | Get-MailboxFolderStatistics -IncludeAnalysis | Sort TopSubjectSize -Descending | Select Name, TopSubject, TopSubjectSize -First 5

Choosing the method

The three native options are cutover, staged and hybrid. Cutover moves everything in one batch and suits organizations under a couple of hundred mailboxes that can accept a single weekend switch. Staged is for Exchange 2003 and 2007 only, so it rarely applies any more. Hybrid keeps directory synchronization and moves mailboxes in batches over weeks, with mail flowing between both sides the whole time.

If you already run Entra Connect, or plan to keep Active Directory as the source of identity, hybrid is almost always the right answer even for small organizations. The mailbox moves are online, users keep working during the copy, and the final switch for each batch takes minutes. The price is a working Hybrid Configuration Wizard setup, which we cover in a separate post.

Building a timeline you can keep

A realistic plan for a company of 50 to 300 mailboxes usually runs six to ten weeks from kickoff to the last on-prem server being shut down. The calendar is driven by three things: the DNS TTLs you have to wait out, the number of nights available for data copies, and how long you need for the pilot group to surface problems.

Put the pilot early and give it a full week. Pick users from different departments who use shared mailboxes, mobile devices and delegated calendars.

1. Week 1: discovery, licensing decision, tenant creation, domain verification, identity sync.
2. Week 2: hybrid or endpoint setup, lower DNS TTLs, first test mailbox move.
3. Week 3: pilot batch of 5 to 10 mailboxes, fix Outlook, mobile and relay issues.
4. Weeks 4 to 7: production batches by department, each finalized on a weeknight with helpdesk staffed the next morning.
5. Week 8: MX cutover, public folders and shared mailboxes, SMTP relay devices repointed.
6. Weeks 9 to 10: monitoring, cleanup, and decommissioning of the on-prem server.

Frequently asked questions

Takeaway

A migration that is measured before it is scheduled rarely goes badly. Inventory the mailboxes, the devices that relay through Exchange, and the people who can change DNS, then size the copy from a real test move. If you want a second set of eyes on the plan before you commit to a date, RackLedge does this work for companies across the Houston area and remotely.