A bit of explenation:
What Is A Reverse PTR Record?
PTR record or more appropriately a reverse PTR record is a process of resolving an IP address to its associated hostname. This is the exact opposite of the process of resolving a hostname to an IP address. Example, when you ping a name mail.somedomain.com it will get resolved to the ip address using the DNS to something like 192.168.1.5. Reverse PTR record does the opposite; it looks up the hostname for the given IP address. In the example above the PTR record for IP address 192.168.1.5 will get resolved to mail.somedomain.com.
Why Do You Need A Reverse PTR Record?
The most common use for looking up a PTR record is done by spam filters. Concept behind this idea is that fly by night spammers who send e-mails out using fake domains generally will not have the appropriate reverse PTR setup at the ISP DNS zone. This criterion is used spam filters to detect spam. If your domain does not have an appropriate reverse PTR record setup then chances are most e-mail spam filtering software will block e-mails from your mail server.
How can you host multiple domains on a droplet, that are expected to send emails?
Once you got a mailserver running, you could setup a PTR record for that mailserver since its sending/receiving all the mails. All the other domains you would like to use for mail, will have to connect to your main mailserver and use that to send and receive mails from. To do this you can find alot of tutorials online, or you could use some sort of CP wich has integrated mail.