Your best choice - Virtual domains
More stuff to read.
Even more stuff to watch.
UPDATE as requested.
Mail server can handle e-mail traffic for multiple domains. This technology is called "Virtual domains". Simply saying, mail server (Postfix) can be configured to accept e-mails for different destination domains. There is setting in the configuration which tells e-mails for which destination mail server should accept.
Simplest setup assumes that mail server for domain example.com would accept e-mails only for example.com. We can expand it to accept e-mails for example1.com, example2.com and super-example500.com.
How server is handling these e-mails for different destinations - will it deliver them to local MDA or relay it to another server - another question.
And there are different ways of setting it up depending on your software. Google for "E-mail virtual domains" and this will bring you a lot of helpful stuff.
Additionally I will say - you will have to tweak not just mail software configuration, but DNS records. MX record defines which servers handle e-mails for particular domain.