Scenario: You receive the following error message when you attempt to onboard a mailbox from Exchange On-Premises to Exchange Online:
“You cant use the domain because it’s not an accepted domain for your organization”
Solution: Check their Email aliases (Get-mailbox steve | Select -expandproperty emailaddresses). The user account may have an invalid email alias OR an alias that is not an accepted domain for the Exchange Online tenant. For Example, the On-Prem mailbox may have an alias of steve@xyz.com but the @xyz.com is not an accepted domain in the cloud. Either add that domain to the cloud OR remove the alias.
Ex-Shell 