How can I run one mailserver on one droplet to handle mail for three (or more) different domains?

My droplet is Ubuntu 14.04. Virtual hosts on Apache have been setup and work fine (even with Ghost self-installed!). I am wanting to set up an email server, likely following the tutorial here for iRedMail or Postfix et al.

I am however confused by how to set up a FQDN for one email domain when I have multiple domain names on one droplet, the same droplet I will install the mail server on.

When I check hostname I get the name of my droplet, MyDroplet. When I check /etc/hosts I see 127.0.0.1 localhost 127.0.1.1 MyDroplet

Now, I have read all the other questions about FQDN but I haven’t been able to understand if the answers apply to my setup since I (apparently) did not see a similar setup.

I have one droplet, with its IP, 123.45.67.89, and the droplet’s name MyDroplet. I have domain1.com and domain2.org and even domain3.net.

So the question is how can I run one mailserver on one droplet to handle mail for three (or more) different domains?

Thank you!