Old and new email

I frequently send emails to one specific recipient, who is external. This recipient used to have an email, let's call it email address removed for privacy reasons. Since then, the domain changed for business reasons, and now the recipient's email is, let's say, email address removed for privacy reasons.  So email address removed for privacy reasons is the preferred and correct email address.

The problem is that when I email email address removed for privacy reasons, the recipient sees the email come through the old inbox, "@x.com" which is then rerouted to the new/correct inbox. So he still does get the message, but I want to make it right and take out the old email completely.

When he replies all to any messages sent to him, the sender is email address removed for privacy reasons, but it automatically copies email address removed for privacy reasons. Does the issue reside in my (internal) system or the recipient's (external) system? Has anyone seen this and can please provide any guidance?

Thanks in advance.