In the attached picture there is an arrow pointing left (replied) and a green one pointing right (forwarded). There's a similar display in Table View. But sometimes the arrows don't appear. In those cases, Ctrl+Shift+O to open the message in conversation to see the received and sent messages in the thread.

Thanks sfhowes, Is there any indication on an open email or just in the mailbox list?

No, the icons only appear in the list, and sometimes it depends on the mail provider.

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