Following this tutorial How To Set Up a Host Name with DigitalOcean (https://www.digitalocean.com/community/tutorials/how-to-set-up-a-host-name-with-digitalocean), my droplet is setup as:
Droplet IP: 188.8.131.52
You should be fine setting up the hostname record you offered. You will also want to likely name the droplet mail1.example1.com as well, so your PTR shows up as that instead of example1.
The PTR of your IP addr...