I migrated users to Office 365 from on-premise Exchange all went well except that when someone from on-premise emails the migrated user using the old email address, the email is not routed at all. But if the address is in the O365 format, the email is delivered to the migrated mailbox in O365. It looked like the onpremise email address is no longer recognize and only the O365 email address is routing email. Is there any settings I should check to make sure that mail routing will take place using the on-premise email address. As mentioned the email address using O365 works fine but not the on-premise email address. No NDR as well. Any suggestions is greatly appreciated.