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 https://learn.microsoft.com/en-us/microsoft-365/compliance/set-up-an-archive-and-deletion-policy-for-mailboxes?view=o365-worldwide
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.
WD
==
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.