What's the difference between "Reply" and "Sender"? If I do "reply", I would do the same as by clicking on "Sender", this is confusing.
This is a bug and enhancement request tracker, here, not a support forum where you can post your questions or request documentation about features. Your ticket will be closed. FYI, there are many cases where Reply and Reply to Sender behave differently: Reply does reply to the From: address, and Reply to Sender does reply to the Sender: address if any (or falls back to the From: address). You're apparently just not aware of what the Sender: header is, maybe never encountered it?