Fulcrum Blog

Article: Office365 Email Tenant-to-Tenant Migrations

Nov 4, 2019 | blog |

by Cindy McManus & Sonny Singh

Consulting Architects

Cindy McManus and Sonny Singh are both Consulting Architects at Fulcrum Technology Solutions. McManus has been a part of the Infrastructure team since 2012. Singh first joined the Infrastructure team in 2013, and then came back in 2017. 

Your company finally finished the email migration to Office365. You’re set. And then your company is bought or buys another company.  The other company is on Office365, too. Great, you’ll just migrate the mailboxes from tenant to tenant.  Not with Microsoft native tools you won’t. There aren’t any. What are your options? That depends on the number of mailboxes to migrate.

A small number of mailboxes can use a “big bang” cut-over. The mailboxes are slated for migration overnight or over a weekend. The email domain is cut over to the new tenant and users begin using the new mailbox. (Any mail queued for delivery during the cut-over will deliver when the email domain is available on the new tenant.) Third party tools are out there that make the migration easier by staging older email first and finishing up the migration with a final pass for the “big bang.” How many mailboxes can you do in a “big bang” cut-over? That depends on how much post-migration support the users need. Do they need help configuring a new Outlook profile? Do they just need the new URL for Outlook? And don’t forget about email configuration on their phones!

If you have enough mailboxes that your migration will extend for weeks or months, you’re going to need a third party tool. Third party tools can easily do the mail migration activities. What happens with the email domain name tells the rest of the story.

Migrations where the email address changes to the new tenant are easier than those where the email address doesn’t change. When the email address changes, the new mailbox has the new address. Email coming to the old address can be forwarded from the old mailbox to the new mailbox. Once the migration is complete, the email domain can be cut over and the old email addresses added to the new mailboxes as a secondary proxy address.

Migrations where the email address remains the same are the ones that will cause grief. The email domain can only exist on one tenant at a time. That’s where a third party tool comes in. There are tools with services that will receive the incoming messages for the email domain and deliver them to the right mailbox in the right tenant.

Deciding how to proceed is the easy part. Once that decision is made is the hard part: planning and execution.


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