Emails failing to Mailbox after primary SMTP change?

I changed a user mailbox primary SMTP to accommodate a married name. Her maiden name still exists as an alias on the mailbox. However, any replies to email threads her old address is on bounceback to the sender. This is internal and external threads. When I tested emailing both her internal and external addresses individually, they deliver fine.

A new Outlook profile was created when we switched over her primary SMTP so her primary sending account is the new one. Any help is greatly appreciated. Thanks in advance.