Reverse DNS, or a PTR record, should be set to your hostname (this is automatically attempted when you setup your Droplet), not to your domain name and you should setup a proper hostname both through the DigitalOcean CP and on your server if you intend on sending and receiving e-mail, if you have not already.
To do this, you'd need to login to the DigitalOcean CP and make sure your Droplet has a proper hostname, such as:
If this is not the case currently, you can click on the name of the Droplet, click on Settings from the left side menu and finally, click on the tab "Rename."
anyname.yourdomain.ext (of course, replace this with your preferred hostname) and click on the blue rename button.
Once set, you'll need to login to the CLI and execute:
To setup the hostname at the server level (this can set or change the hostname, in either case, it sets the hostname to whatever you pass).
Once you've done this, then you'd need to make sure you've setup an SPF entry and an A entry for your hostname. The A entry would like something like:
A anyname PUBLIC_DROPLET_IP
You don't need to enter the full hostname for the A entry, just the name (as shown in my example above). You'll need to replace
PUBLIC_DROPLET_IP with the IP of your Droplet.