The main mail protocol, SMTP has a hole in it: there is no verification of the sender. This allows spammers and others to pretend to be anyone they want. But with a little bit of effort, sendmail (for example) can be configured to use SSL and user verification. That way you can have your own SMTP-server accessible anywhere, without open relay.
There are also several new protocols for fixing SMTP problems:
Set it up
Now you do not want anyone to be able to use your mail server as spam-catapult. So you need sender verification in your postfix service.
Make sure your authentication service is running:
has to deliver
In /etc/postfix/main.cf, set
smtp_sasl_auth_enable = yes smtpd_sasl_auth_enable = yes
To prove it works, use kmail. Set "server requires password" in the outgoing mail account.
If you get an error message like
Error: authentication failed: generic failure
Make sure the saslauth daemon is running: