Send from Exchange Online while MX points to on-prem?

Question

We have an existing Exchange Online tenant with users synced from on-prem AD. We have acquired another organization (only ~50 users) that operates their own on-prem Exchange server. They will be keeping their own on-prem AD domain for the foreseeable future, but migrating all of their mailboxes to our Exchange Online tenant, keeping their current email domain. This is my plan to migrate their users over:

1. Add their email domain to our EO tenant. Publish domain verification TXT record and update their SPF record to include EO.
2. Create new user accounts and remote mailboxes in our domain for one department at a time, probably 10-20 users.
3. Forward those user's on-prem mailboxes to their new EO mailboxes.
4. Get the users set up to use the new accounts.
5. Export data from on-prem mailboxes and import to EO.
6. Repeat steps 2-5 until all users, shared mailboxes, resources, and distro groups are migrated.
7. Publish new MX record for EO-only.
8. Shut down old on-prem servers after waiting a suitable safety interval.

Does anyone see any obvious problems with this? I expect the whole process to last less than a week.

Answer 1

Hi @JayCarper-5747

I have noticed your feedback and did more research.

With your requirements, you can use the **Exchange cutover method **to migrate all mailboxes, contacts, and distribution groups from your on-premises Exchange organization to Exchange Online in a single batch.

This method requires you to verify your domain ownership, create a migration endpoint, and start the migration batch.

Details please refer to https://learn.microsoft.com/en-us/exchange/mailbox-migration/cutover-migration-to-office-365

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Answer 2

Hi @JayCarper-5747

Thanks for posting in our Q&A forum.

According to my experience, since your on-prem organization have only less than 50 users, it is recommended to perform with Exchange-based hybrid deployment. Then we can move mailbox to EO smoothly.

To prepare the hybrid deployment, please refer to: https://learn.microsoft.com/en-us/exchange/exchange-hybrid

Before migration, we'd better to make sure the version of Exchange server is the latest one.

You also can suggest end-user backup their data if needed via exporting from Outlook client and saving it as .pst file before you deploy the hybrid. 

Reference: Move mailboxes between on-premises and Exchange Online organizations in hybrid deployments

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