I have marked one incoming email in my inbox as unread. After that, I see “1” badge not only in the Inbox folder but also in the Sent folder as well.
The conversation thread includes both incoming and sent emails. However, the only email marked as unread in that thread is the incoming message, not any of the sent emails.
If the conversation is made up of messages in multiple folders and one of the messages in the conversation is unread, the indicator will show on any folder where the conversation exists.
If conversations don’t work for you, you can disable them using Menu > View > Conversations > Disable Conversations.
