My server is vulnerable on port 25 (SMTP with out auth)

==== Required information ====
- iRedMail version: 0.8.6
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Linux/BSD distribution name and version: Debian Squeeze
- Related log if you're reporting an issue:
====

Using port 25 and telnet from my server anyone could send emails without the need to login, that is, anyone could send emails pretending to be someone else.

How I can I avoid this.

Thanks for all, I hope someone can help me.
Best regards.

PS: Sorry for my english, it's not my primary language.