Sent Icon in Inbox

We have a user who is using Lotus Notes 9, with a Template Standard 8. He has a forwarding from his old address to his new one. Someone sends him a mail on his old address, it gets forwarded to his new one. When he replies back, it is from his new address but the old one appears in Cc. A mail comes to his inbox. Which I guess is quite normal as his old address was in Cc. But the icon that shows up in Inbox is the ‘Sent envelop icon’! When he deletes it from Inbox, the Sent item also gets deleted! How come is it related?

Thanks to help me out please.