The ResKey templates are a wonderful feature. I have 2 set on "auto send": 1) a confirmation email stating that a deposit has been charged, our check-in times, and a receipt showing the balance due, then 2) Three days before arrival, I send a "see you soon" email with a second reminder of our check-in times and a list of sights they can see if they arrive early.
I have a few templates set up that I manually send: 1) Confirmation of a cancellation 2) Late check-in instructions for guests arriving after 9pm 3) Manual reservation confirmation for reservations taken over the phone that includes all of our cancellation policies, check-in & check-out times, and house rules (no pets, no kids, no smoking). With phone reservations, I require that the guest send a reply confirming that they've read this email so at least I have something acknowledging our cancellation policies if a chargeback occurs -- hasn't happened yet, but CYA.
I also have a "Thank you" email that I manually send. I have a template, but I add personal items to it like what they did during their stay, if they gave my dog all sorts of attention, where they're headed on their travels if they're driving through, etc. I include a link to Google reviews. Google reviews have a big impact on your SEO