How to make imported mailboxes map properly to the existing accounts

Lilly Hill 60 Reputation points
2025-10-29T10:21:20.55+00:00

I have a tenancy set up with all users created and using .onmicrosoft email addresses to allow them to use the licenses for OneDrive and Teams.

I am trying to migrate the emails from an on prem server into the tenancy.

I set it up so every account had the matching .onmicrosoft email address as an alias on the on prem server.

I set up a cutover migration. Some of the accounts have migrated but have created new accounts in the 365 tenancy so the licenses aren't applied to the mailboxes and there seems to be no way to merge these together. Users have a significant amount of data in OneDrive and an entire SharePoint set up with permissions so I can't just swap the licenses to the new accounts.

The rest of the accounts have refused to migrate with the error MigrationProvisioningPermanentException: The proxy address "smtp:<Removed PII>.com" is already being used by the proxy addresses or LegacyExchangeDN which makes no sense, of course it is already in use. It is supposed to be importing into the in use account.

Can anyone recommend a way to map the accounts properly?

I'm not in a position to use the azcopy import feature in purview as the amount of data that needs to be moved will take weeks to download and upload, by which time it will all be out of date. I need it to sync in the way it is supposed to in a cutover migration.

Exchange | Other
Exchange | Other
A powerful email and collaboration platform developed by Microsoft, designed to support enterprise-level communication and productivity. Miscellaneous topics that do not fit into specific categories.
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  1. Austin-H 6,175 Reputation points Microsoft External Staff Moderator
    2025-10-29T23:10:37.0233333+00:00

    Hello Lily Hill
    Thank you for posting your question in Microsoft Q&A. 

    Please note that our forum is a public platform, and we will modify your question to hide your personal information in the description. Kindly ensure that you hide any personal or organizational information the next time you post an error or other details to protect personal data. 

    We understand you're planning to migrate user mailboxes from an on-premises environment to the cloud. Based on our research, Cutover Migration is typically suited for scenarios where no users exist in Microsoft 365 yet. It attempts to create mailboxes in Microsoft 365 and, when it finds a matching primary SMTP address, it tries to provision a mailbox for that existing user. 

    However, since you've already created cloud-only user accounts and assigned services like OneDrive and SharePoint, this approach may lead to issues such as: 

    Mailbox creation conflicts:  It created new mail-enabled users or mailboxes because it couldn't properly match and "mailbox-enable" the existing cloud user object.  

    Proxy address conflicts: Issues like "MigrationProvisioningPermanentException: The proxy address is already being used..." indicate that the migration is attempting to create a new mailbox using a primary email address that is already assigned to an existing cloud-only user. This conflict prevents successful mailbox provisioning. 

    Could you try to change to Hybrid Migration (using Azure AD Connect for directory sync + Exchange Remote Mailbox Moves), this is the suitable method for scenarios like yours: pre-existing M365 users, on-prem Exchange, and the need for seamless merging without data loss or downtime. It: 

    Maps accounts properly: Syncs your on-prem AD users to match existing M365 objects via "soft matching" (UPN/proxy addresses) or "hard matching" (ExchangeGUID). 

    Merges data: Moves the mailbox content from on-prem into the existing M365 user's Exchange Online (EXO) mailbox, preserving the user's identity, licenses, OneDrive, SharePoint permissions, and Teams setup. 

    Incremental sync: Initial full sync + deltas every 24 hours (up to 90 days), minimizing staleness, far better than AzCopy's one-time bulk transfer. 

    Supports up to unlimited mailboxes (unlike cutover's 2,000 limit) and works with your setup. 

    You can refer via: Use Minimal Hybrid to quickly migrate Exchange mailboxes to Microsoft 365 or Office 365 | Microsoft… 

    Please understand that our initial reply may not always immediately resolve the issue. However, with your help and more detailed information, we can work together to find a solution. 

    If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".      

    Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread. 

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  1. Diane P 7,635 Reputation points Independent Advisor
    2025-10-29T13:16:17.63+00:00

    Remove the onmicrosoft addresses from the accounts you are migrating from.

    Have you verified the domain? Add the permanent address to the accounts and then set up the migration. But even if you don't, you map the addresses in the excel sheet that contains the usernames and passwords.

    You won't be able to merge the accounts that did not migrate properly. The mail should still be on the other server so you can delete those mailboxes from Microsoft 365 and try again.

    This is Microsoft's support article:

    https://video2.skills-academy.com/exchange/mailbox-migration/mailbox-migration?WT.mc_id=M365-MVP-6934

  2. Diane P 7,635 Reputation points Independent Advisor
    2025-10-31T13:22:57.6033333+00:00

    When you set up the CSV in Excel, did you use the new mailbox's default SMTP address as the mailbox to import into?

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