Is this a server with ISPConfig installed? Your main.cf does not look like you have ISPConfig installed. Did you follow exactly the perfect setzp guide to install your server?
And one other question, what do you tell programs your smtp server is when they want it send email? mail.yourdomain.com? Then a username and password of an account in that domain you own? (such as the logins to firstname.lastname@example.org)?
You may use any domain that points to your server as smtp and pop3 server in the mail client.