9 Prerequisites for Large Exchange Online Migrations
A Large Exchange Online Migration involves thousands of mailboxes, complex identity dependencies, and large volumes of historical email. While the migration process may take days /weeks, the success of the project is usually determined long before the first mailbox is moved.
Prerequisites for large Exchange Online migrations begin with a thorough environment inventory, ensuring Active Directory and Entra ID are ready and planning for mail flow and DNS. Documenting distribution groups, dynamic distribution groups, and mail-enabled contacts is critical. Also identify applications and devices that use on-premises SMTP relay.
Key Takeaways:
9 Prerequisites for Large Exchange Online Migrations
The Enterprise Exchange Online migration planning phase helps identify problems early, prepare for the actual migration, and minimize downtime. Licensing, network, security, and other configurations are ready, and users are prepared for the transition.
In short, proper preparation improves the chances of a smooth, risk-free, and successful migration.
Let us list these nine prerequisites.
1. Accurate Exchange Online Licensing
Check the plan of each mailbox, Exchange Online Plan 1 and Plan 2 offer different mailbox capacities, archive capabilities, and compliance features. This extra archive space grows slowly depending on the plan. Likewise, some plans include additional advanced compliance tools if you decide to put a mailbox on hold. Review these details carefully before deciding.
2. Size of Mailbox and Item Count
Exchange Online mailbox limits dictate how mailbox migration can happen; there is a limit of 10,000 direct child folders within any single parent folder, and Microsoft recommends no more than 1,000,000 messages per folder. Individual message size is also capped at 35 MB for sending and 36 MB for receiving.
Public folders and space are capped as well: There are multiple built-in combinations in the Microsoft environment, and therefore it is key to check all of this before the process of migration.
Planning a migration of 1,000+ mailboxes? Get a quick quote now.
3. Planning Network Bandwidth
First, test your actual internet speed. Large enterprises use extra security software that usually slows speeds but runs in the background and isn’t obvious.
The best email migration service provider will help you estimate what you need; it is usually calculated by accounting for your data/time available and any overheads.
4. How Ready is the Microsoft Graph API Throttling?
Microsoft API throttling limits the number of requests you can send to Microsoft services within a specific period. In large online exchange migrations, this helps prevent excessive resource consumption that could eventually cause service degradation or downtime.
Throttling helps ensure users maintain access to Microsoft services during high-traffic periods. Microsoft API throttling is critical to maintaining the performance and reliability of Microsoft 365 services during large-scale migrations.
5. Optimize Mailbox Batch Sizing
The optimal number of mailboxes to migrate at the same time depends on Microsoft’s speed limits. There is no single formula or rule, as it depends on mailbox size and how close you are to hitting Microsoft’s limits. One tip is to migrate only new or changed data on repeat passes, rather than re-checking everything from scratch each time.
6. Configuring DNS and Mail Flow
Before complete data transfer starts, ensure that the technical settings that control email delivery are in place-these are small text records tied to your domain name. Errors in these settings can cause emails to bounce back or fail delivery for everyone at the company at once.
Check a setting called “TTL” ahead of time and lower it-this controls how quickly changes to these records spread across the internet. If done early, the actual switch-over day can be faster. Verify if the company’s official domains and backup email addresses are set up correctly beforehand.
7. Hybrid Identity Readiness
How are employee logins maintained? Are logins maintained in the cloud, or are they still partially synced from their on-site servers? This decision affects new account creation and logins after the move, so we need clarity. If you use a mixed approach (on-site and cloud), double-check that the Microsoft tool used to connect the two has run successfully.
8. Requirements of Compliance, Retention & Litigation
Identify mailboxes with special legal or compliance requirements ahead of migration. Additional features may be needed, so plan the right license (Plan 2, or Plan 1 with an add-on), as discussed in point 1.
Remember, old emails can be moved to archive storage only once the mailbox passes 10 MB; do not assume small mailboxes are automatically handled the way you expect.
9. Pilot Migration Testing
A good approach to carrying out pilot test runs is to consider 5 to 10% of your total users, a group that includes a mix of resources: IT staff who can give detailed feedback, one senior executive (as their mailboxes tend to be larger and more complex), and resources who use shared mailboxes/ who relies heavily on shared folders.
If this test group runs into issues, it means you found those problems before they became bigger.
How Does CloudFuze Handle Large Exchange Online Migrations?
CloudFuze Migrate is an email migration platform built to move files, emails, chats, and entire tenants between major cloud platforms. CloudFuze handles large Exchange Online migrations and starts with licensing checks, DNS validation, identity readiness, and end-to-end pilot coordination systematically. It begins with licensing checks, DNS validation, identity readiness, and end-to-end pilot coordination, backed by SOC 2 Type 2, ISO 27001, and GDPR compliance.
Data fidelity is preserved -permissions, timestamps, folder structures, and metadata are transferred intact, without any manual mailbox-by-mailbox verification later. Executing Delta migration keeps the source and destination in sync after the initial move, reducing the pressure to rely on a single, high-risk cutover migration window.
Build the source and destination in sync after the initial move, reducing the pressure to rely on a roadmap, validate prerequisites, and execute a low-risk migration with expert guidance. More importantly, choose the right email migration partner to get started. Talk to our migration experts.
Frequently Asked Questions
1. List the technical prerequisites for a large Exchange Online migration.
Ensure correct Microsoft Exchange licensing, a mailbox size/item count audit, planned network bandwidth, Graph API throttling readiness, batch sizes, DNS/mail flow configuration, hybrid identity setup, compliance/retention requirements, and pilot migration. Verify all of this in advance.
2. What licenses are needed before starting a large Exchange Online migration?
Exchange Online Plan 1 (50 GB mailboxes) or Plan 2 (100 GB, plus a 1.5 TB auto-expanding archive). Litigation holds, and advanced compliance features require Plan 2, or Plan 1 with the Archiving add-on.
3. How to avoid API throttling errors in a large-scale Exchange Online migration?
Respect Microsoft Graph throttling guidance, honor Retry- After headers, use migration tools like CloudFuze Migrate that automatically manage request pacing and retries. For persistent slowdowns, one can also request a temporary EWS throttling limit increase from Microsoft.
4. What is the ideal batch size when migrating thousands of mailboxes to Exchange Online?
Validate mailbox size, folder counts, item counts, and message size limits against current Exchange Online service limits before migration. The right number per batch depends primarily on mailbox size distribution and current throttling headroom.
5. How should IT teams plan hybrid identity (Entra ID Connect) before an Exchange Online migration?
Confirm cloud-only vs. hybrid sync before migration starts, and in hybrid setups, verify the Hybrid Configuration Wizard ran successfully, and connector tests pass; resolving identity conflicts mid-migration is far harder than confirming setup upfront.
6. What causes migration throttling in Microsoft 365?
Migration throttling occurs when excessive requests are detected that could impact Microsoft 365 service performance. These could be due to large mailbox batches, high concurrency, and aggressive retries -making controlled migration waves and having throttling-aware tools are therefore essential.
7. How many mailboxes should be migrated in a single batch?
There is no right or fixed size. The optimal number depends on mailbox size, available bandwidth, and current throttling conditions. Enterprises should hence start with pilot batches and scale based on actual migration performance.
