Additionally, legacy dedicated users receive the following non-delivery report (NDR):
To avoid any mail delivery problems, the targetAddress value for the object in the on-premises AD should not be changed immediately after the mailbox is moved. This is because the value that's set in targetAddress after the move to vNext is the only domain.onmicrosoft.com address that will exist on the vNext mailbox. If a new secondary SMTP address that has the domain.onmicrosoft.com suffix is added to the object, that address is not synchronized to the vNext mailbox.
The targetAddress attribute is used for routing. It's not exposed in an email message that's sent to the user. The address of the email that's sent by the user will be the value that's set in their PrimarySMTPAddress property.
Article ID: 3202871 - Last Review: 10 Nov 2016 - Revision: 1