Spam mail through SMTP and user spoofing

view story

http://serverfault.com – I have noticed that it's possible to telnet into a mailserver that I own and send spoofed messages to other clients. This only works for the domain that the mail server is regarding; I cannot do it for other domains. For example; lets say that I own example.com. If I telnet example.com 25 I can successfully send a message to another user without authentication: HELO local MAIL FROM: [email protected] RCPT TO: [email protected] DATA SUBJECT: Whatever this is spam Spam spam spam . I consider this a big problem; how do I secure this? (HowTos)