When composing an email, you'll notice a new checkbox: "Remind me to follow up if no reply in [days]".
Simply check this box and choose your timeframe, and we'll keep an eye out for an email reply from any of the recipients. If you don't get a reply, then a reminder will show up in your Inbox on the given day.
If you do get a reply in time, we won't bother you with any reminders.
Once a follow up is triggered, like any other Inbox item, you can then mark that follow up as Done or Snooze it for later, if you choose.