I am having trouble configuring my mail server (Dovecot and Postfix). The problem is that someone can send spam mails to my address from my own mail server. The server is not open-relay but it seems to be half-open.
Bad Guy sends spam from:
email@example.com to firstname.lastname@example.org
email@example.com to firstname.lastname@example.org
without needing any password.
Sending spam to others is not possible. I tried sending a mail to my own domain from my own domain via Telnet and it was possible without password. How can I prevent this?