ExPro: Error: "Mailbox move to O365 fails with "You can't use the domain because it's not an accept" (4303299)

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ExPro: Error: "Mailbox move to O365 fails with "You can't use the domain because it's not an accept"
Description

A mailbox move request did not start due to this error in the application event log, "You can't use the domain because it's not an accepted domain for your organization.
Cause

This is due to proxy addresses in the mailbox you are trying to move and have domain names which have not been verified in Exchange Online (M365).
Example: @contoso.com isn't verified domain in M365.
This domain was not registered in Office 365 (M365).
Resolution

Confirm all the Proxy Addresses in your source are listed as Accepted Domain in your target M365.
Additionally, the error typically occurs when M365 Proxy Server isn't define within Sites located Forest tab | Sites

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