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Office 365 Cutover MIgration from Exchange 2010 "The name is already being used"

Last Modified: 2017-04-13
Hi all,

Having problems with a cutover migration from Exchange 2010 that I could use some help on please.

We have an organisational account in 365 with 5 users set up which were only used to get the software so are on .onmicrosoft.com email addresses.  We are now looking to migrate those users and the rest of the company into 365.  I've imported the domain into the account (but not changed the DNS settings yet) and set up a cutover migration from their existing Exchange 2010 server.  Problem I have is that the migration failed for all users that already had a 365 .onmicrosoft account set up prior to the migration.  For example one user called john.smith had a 365 account of john.smith@ourclient.onmicrosoft .com and a domain account/email address of john.smith@ourclient.com  So when we ran the migration that one failed with "the name is already being used" so I deleted the .onmicrosoft user in 365 and tried again but still getting the same error.  I don't understand where it is finding this name if the user has been deleted.

Would really appreciate some help with this.

Many thanks

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Most Valuable Expert 2015
Distinguished Expert 2019

The easiest way to check is to run the following cmdlet against Exchange Online:

Get-Recipient john.smith@ourclient.com

Open in new window

It should return any matching objects, which you can then delete or simply change their attributes.

Also, the Cutover migration process provisions accounts based on the value of the "mail" attribute of the on-prem object, so check for that too.
Senior Network Systems Specialist
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Thanks Peter I've run that command and will try the migration again now and let you know.


Hi, I'm afraid that didn't work, still fails on that mailbox with the same error. I've checked deleted users in the admin console and it's empty. Still says "The user john.smith is already being used"

Would really appreciate any further help with this




Hi Peter, that didn't fix my original problem but did at least get rid of the user so awarding you the points, many thanks for your help.
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