Postfix seems to allow anyone to send emails to domains that are configured in ISPConfig 3.
220 server1.mydomain.co.uk ESMTP Postfix (Debian/GNU)
MAIL FROM: email@example.com
250 2.1.0 Ok
RCPT TO: firstname.lastname@example.org
554 5.7.1 <email@example.com>: Relay access denied
RCPT TO: firstname.lastname@example.org
250 2.1.5 Ok
354 End data with <CR><LF>.<CR><LF>
250 2.0.0 Ok: queued as 62C57876E
As you can see, it allows the user to send emails from any domain, to my domain without them having to login.
Can anybody help me?