I have multiple domains and I want to install postfix to send emails only. How can I set it up on my vps?
During the install process (sudo apt-get install postfix) it will ask you, in an ncurses text-mode GUI, to "select the mail server configuration type that best meets your needs". The correct answer here will be either "Internet site" if you want the machine to deliver mail directly, or "Satellite system" if you want it to relay all mail through a single smarthost instead.
Beyond that, you'll have to be more specific about what you want - should the machine listen for INCOMING smtp traffic, or will it only be relaying mail out from localhost? Should it accept unauthenticated SMTP, and if so, from whom? Should it accept authenticated SMTP traffic - only, or from all but a certain trusted range?
All these things are possible, but we need to know more about what you want to do.
Please consult the postfix documentation for details:
For the sending of mails via smtp it is necessary that you configure at least: