There’s a lot of work necessary to set up an e-mail host that will pass through all of the filters that the “big guys” (GMail, Microsoft, etc.) set up to prevent spam. If you use a service like Sendgrid or Mailgun then you don’t have to go through as much trouble.
Sendgrid has a free plan and documentation on how to configure Postfix: https://sendgrid.com/docs/for-developers/sending-email/postfix/
Mailgun is also free for low-volume uses. Tutorial: https://blog.birkhoff.me/send-email-thru-mailgun-on-centos-7/