How to set up a VPS server for sending E-mail in order to use it as SMTP?

Question

SMTP

It is necessary to set up a VPS server for sending E-mail in order to use it as SMTP.
how to do it?

Answer 1

At least the OS is indicated, for deb based something like this:

apt-get install exim4 exim4-config
dpkg-reconfigure exim4-config

Answer the wizard's questions on how to use and who should be allowed to use the server.

Answer 2

The same can be done on postfix.
Make a default config in which to change only hoostname and so that the daemon listens on an external address.
Through iptables, open port 25 only for the server from which the mailing will be initiated.