1. Login to the new Exchange server and open Exchange Management Console
2. Browse to Microsoft Exchange On-Premises > Recipient Configuration > Mailbox and find the user's mailbox you wish to change.
3. Under the E-mail Addresses tab, uncheck the Automatically update e-mail addresses based on e-mail address policy checkbox.
4. Add the new SMTP address, or right click the address you want as primary and choose Set As Reply.