Questions about IMAP migration


A migration batch and a migration endpoint have been created.

The synchronization also works so far:

1) Although the default retention policy applies to the mailbox (Delete deleted items and delete junk if older than 30 days), there are also much older emails in the Sent Items folder.

The junk folder also contains at least one mail that is older than 30 days.

my question :

When is the retention policy always carried out?

Will it even be carried out while the IMAP migration is ongoing?

2) What about migration via migration batch with duplicates? Are they recognized by the system and not imported or are they imported with. We cannot see any systematics here.

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  1. Walisson Dias 316 Reputation points

    Hi @Anonymous ,

    1) After you apply the new retention policy to mailboxes, it can take up to 7 days in Exchange Online for the new retention settings to be applied to the mailboxes. This is because a process called the Managed Folder Assistant processes mailboxes at least once every 7 days. Instead of waiting for the Managed Folder Assistant to run, you can force this to happen by running the Start-ManagedFolderAssistant cmdlet in Exchange Online PowerShell.

    Mode details here

    2) If you try to add an email that is already migrating, you will receive an error message that the email is already migrating in another batch

    Hope this helps.


    Please "Accept the answer" if the information helped you. This will help us and others in the community as well.

  2. Joyce Shen - MSFT 16,646 Reputation points

    Hi @Anonymous

    Looks like here is a related thread discussed the similar question: Recent email migration to office 365 and retention tags aren't archiving.

    In migration, the retention age is calculated when the emails moved to Office 365 service. For example, the emails are migrated to the Office 365 mailbox on 11/01/2015 via IMAP migration and the retention tag is to move to archive after 2 years, the emails will be archived at 11/01/2017 in this case.

    And the official document introduces about: Items reappear in a user's mailbox or are duplicated in the archive mailbox when an archiving policy is enabled during IMAP migration

    In Microsoft 365, users may experience one or both of the following symptoms:

    • Items that are expected to be archived reappear in their inbox in Microsoft Outlook and Outlook Web App.
    • Items are duplicated in the archive mailbox.

    To fix this problem, use one of the following methods:

    • Disable the archiving policy until the IMAP migration process is complete.
    • Stop the migration batch to prevent more duplicate items from being created. Additionally, you must manually remove the duplicated items from the mailbox. Or, delete the mailbox, and then remigrate it to Microsoft 365.

    If an Answer is helpful, please click "Accept Answer" and upvote it.

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  3. Amit Singh 4,851 Reputation points

    Based on my experience, in migration, the retention age is calculated when the emails are moved to Office 365 service. So, for example, the emails are migrated to the Office 365 mailbox on 11/01/2022 via IMAP migration, and the retention tag is to move to archive after 2 years, the emails will be archived at 11/01/2024 in this case.

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