For many users, this is all one needs to do to configure postfix on an SMTP gateway: Install postfix from a binary package via your local package tool (rpm, etc.) or by compiling from source and ...

As a result, many organizations that run Exchange internally have been opting for an open source e-mail server to act as their SMTP gateway. Common choices include Sendmail or Postfix running on ...

To bridge the time I decided to set up temporary postfix/dovecot on Centos 7 droplet on digital ocean. The server is listening on 22 110 143 465 587 993 995 but not on 25.

Looks like you forgot to do all the edits to the main.cf file outlined in that tutorial. You missed the SASL stuff.-----One thing about Postfix, however, is that you may not use Cyrus SASL.