Postfix and "relay forwarding denied"

I can pick up mail from the server using my mail app on my home machine but when I try to use my mail app to send mail through the server I get the error "relay forwarding denied."

I understand Postfix can be configured for SASL support to authenticate remote SMTP clients to the Postfix SMTP server to allow for trusted clients. And I saw the thread "SMTP Authentication and TLS" that says SMTP Authentication will be coming soon to Virtualmin but is not yet included. Is it true that it is not yet included?

Are there other recommended ways to resolve this issue? Does Dovecot offer a way to send mail?