Emails sent to former employee actually being sent to another domain

We have an employee who left a couple of years ago. His email address user@domain.com does NOT exist on our mail server and he no longer has an AD account. A user on another network contacted us saying emails to user@domain.com were going to him. The strange thing is this guy's email is the same user@domain123.com. So, the user portion of the email is the same as the guy who is no longer here. The really strange thing is that my CTO did a test email from INSIDE our network and sent an email to user@domain.com expecting a bounceback since that user doesn't exist. BUT, the guy at user@domain123.com actually got it and responded!

I have no idea how this can be happening or how to find out how to stop it. Why would our Exchange server accept an email for a user that doesn't exist on it and actually send it to another domain?
scubadiver_daveAsked:
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Benjamin MOREAUProject ManagerCommented:
Maybe you have a contact created on your Mail Server. This contact can accpet mail sended to user@domain.com and redirect to user@domain123.com.
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scubadiver_daveAuthor Commented:
It was indeed an issue with a contact.
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