If this is a valid address then it should certainly not be getting flagged as "User Unknown". I'd recommend submitting a ticket to Adobe Campaign Support as we'll likely need to review the specific recipient to investigate further as it could be an incorrect response coming back into Campaign.
That said, as a workaround, since the address is indeed legit, please try "Whitelisting" it versus placing it in a "Valid" state. Valid only resets the address so it can be resent to but it is still subject to the same scenario. Whitelisting will prevent the address from moving into a quarantined or blacklisted state.