Hi @Jose IT support ,
To narrow down your question, I wonder if you are currently migrating Exchange online mailboxes to Gmail?
According to my search, the way you used to import mails seems to be used to import POP mailbox account into Gmail mailbox.
If it is a Outlook.com mailbox, please refer to the error prompt to open the mailbox to complete verification and change pop username to full address(example@contoso.com).
If you are migrating your Exchange online tenant to the Google workspace, you can refer to the migration method at this link:
Migrate from Exchange or Exchange Online to Google Workspace - Google Workspace Admin Help
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Or you can also export the messages as PST files in the Outlook client and then import them into Gmail.
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