I have the problem with replace sender's email address. My email users can replace sender's email address (FROM on mail's header.
This problem gives possibility for send spam or bad messages from some of my other email users.
$telnet mail.exemple.com 25
Connected to mail.exemple.com.
Escape character is '^]'.
220 mail.exemple.com ESMTP Postfix
MAIL FROM firstname.lastname@example.org
250 2.1.0 Ok
250 2.1.5 Ok
354 End data with <CR><LF>.<CR><LF>
The bad message! :P.
250 2.0.0 Ok: queued as 65040230001
221 2.0.0 Bye
Connection closed by foreign host.
How can we create security?