Telnet Issues for Postfix through port 25

Hello,

First of all I hope this is the right forum to post in, if not can a kind Mod move it and let me know here it has gone.

I need to stop anybody from using telnet to post emails from one of my inteal mailboxes to another or the same one, so for example I can telnet to my server through port 25, issue a "helo" command, then a "mail from:" command, followed by a "rcpt to:" then the "data" command, etc. and the mail gets delivered - and i am never asked for a password, not good if anyone finds this loophole. I need them to login first, but I still need to allow incoming mail. I am using Postfix, Dovecot and Roundcube. I have been trying to configure Postfix to do this for a long time now and need some help!

If I set:

smtp_client_restrictions = permit_mynetworks, reject_rbl_client sbl.spamhaus.org, reject_rbl_client blackholes.easynet.nl

Then anyone can telnet in and send a message, relaying is not permitted - this only works from one inteal mail address to another, but if I do this:

smtp_client_restrictions = permit_mynetworks, reject_rbl_client sbl.spamhaus.org, reject_rbl_client blackholes.easynet.nl, reject

nobody can telnet in, but I caot receive any exteal mail either! This defeats the object of the mail server.

Can someone please let me know what I am doing wrong?

Thanks in Advance, Clock.