A lead is anyone who replies to you, but you can tweak this or manually turn someone into a lead.
When you mark leads as "won," "lost," "ignored," or when you assign them to your teammates Mailshake automatically takes you to the next one. This makes it super easy (and satisfying) to catch up on everything.
If recipients reply back, they're re-opened so they show up in the queue again.
Your entire correspondence and their history of clicks, opens, etc. are always in view because context is key to a meaningful reply.