It goes to the Sent folder, but the other non-Postfix/Dovecot email does not receive the email. The other non-Postfix/Dovecot email can send emails to the Postfix/Dovecot email successfully.
Some more info on this. I am using a .fbx.one domain. Do I need to make changes to some DNS manually? I have not done that. If I do have to do that, then how do you do that?
For the email server to work, you need to have your own domain name, one that you purchase from a domain authority.