Currently you can only reply to the latest email in an Inbox or Help Desk conversation. There’re a huge number of reasons where the ability to reply to an older email is the preferred option. A prime example is where the last email you’ve received is some automatic reply (undelivered, etc.).
Please add a reply button to all emails in a Conversation thread. If you wish to differentiate clearly for users which is the latest email and which isn’t, you could add the reply option to older emails under the … options.
