In order to migrate IMAP mailboxes to Microsoft 365 or Office 365, normally we can refer to the document below:
Migrate other types of IMAP mailboxes to Microsoft 365 or Office 365
Would you go through it and see if the steps have been followed properly?
Besides, for the error you encountered, it's suggested to double check that the value in the EmailAddress column of the CSV file is correctly entered. It should be one of the proxy addresses on the cloud mailbox that you are trying to migrate the data to. See Troubleshooting issues with IMAP mailbox migration.
By the way, from the description, do you only need to migrate this specific mailbox? If yes, it might be simpler to migrate the emails via PST files. You can add the IMAP mailbox to Outlook, export mails of the past month to a PST file, then you would be able to import the PST file to firstname.lastname@example.org via either OWA or Outlook desktop. For more about exporting and importing PST files in Outlook, hopefully you can find the articles below helpful:
- Export or backup email, contacts, and calendar to an Outlook .pst file
- Import email, contacts, and calendar from an Outlook .pst file
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