Using New Outlook on Windows for professional communication and productivity
Hi, I'm Diane and I'm happy to help you today.
Is the old address an outlook.com address? Those addresses are only used to sign in. If so, when you send mail though outlook.com using a non-microsoft address, the From address is not routable. You need to use the correct mail server settings with it.
Let me know how it goes, I'm here to help you further if needed.
-- Diane