Click on the Timeline tab to find record of all communications sent to and opened by the contact, as well as all clicks on properties from your emails.
This is a great resource to check if a contact references "the email you just sent me". As a lot of communications may be automated and sent on your behalf, you can refer to the timeline to determine exactly which email or property they may be referring to.
It's also a great way to confirm if a contact received a particular email, and if they've opened it.
The date range for the timeline can be adjusted (8), and you can also filter by Sent Email, Opened Email or Clicked Property (9).