I'm facing some serious challenges migrating email accounts to Office 365 from an on-premises Exchange. I'm currently working with several companies that have been using licensed Office 365 accounts for services like Teams, SharePoint, and OneDrive, alongside their on-prem setup. I know starting fresh is ideal, but it's a bit late for that now.
I'm executing cutover migrations, but it's been a nightmare. When I set the email addresses of Office 365 accounts to match the on-prem accounts, I find that only half of the migrations complete successfully. The rest fail with an error stating the proxy address is in use.
Alternatively, when using the company.onmicrosoft address for the Office 365 accounts and setting it as an alias to the on-prem emails, the same issue arises; half go through fine while the other half fail due to the proxy address already being used yet again.
I've even tried matching the firstname.lastname section of the email addresses while keeping the Office 365 accounts as .onmicrosoft addresses, but that ends up creating new accounts for users instead of importing data to existing ones.
I can't simply switch licenses to the new accounts because there's years of crucial OneDrive and Teams data, as well as a complex SharePoint permissions structure tied to the original Office 365 accounts, which I cannot afford to lose. Plus, there's no effective way to import that data manually. I can't ask users to pause their email communications for a week either.
Does anyone have advice on how I can successfully map these accounts? I'm absolutely at my wit's end with this situation!
1 Answer
It sounds like you're dealing with a common issue in migrations! It might help to check if the accounts have a bundled license that includes Exchange Online. Sometimes, migrations hit a snag if the target address already has a mailbox in Exchange Online. I would recommend temporarily removing that part of their licenses before you transition the data. Just ensure that those existing mailboxes are primarily holding notification emails, so you should be safe, but double-check with a few accounts first—better safe than sorry!
That's solid advice! I had a similar situation before. Making sure only the right components remain active can save a lot of headaches down the road.