Mailserver slow
 

What can be the cause that I send a mail to my mailserver and that it arrives like 2 hours later?

Also when I stop and start again the postfix daemon, it takes a while (like 30 seconds or so) when a couple of days ago everything was very quick.

OK checking out the logs I see that I was an open-relay :(

Temporarily I just limited the SMTP to the local machine. Anyway, is there any guide of tuning and securing Postfix in ISPConfig once installed?

The best way is to limit SMTP relaying to the IP 127.0.0.1 and the enable SMTP-Auth for your mailusers.

Have you read one of the howtos? In each howto is a section on how to configure postfix for smtp-auth. You will find the howtos on the ISPConfig documentation page or at www.howtoforge.com.

Have a look here: http://www.howtoforge.com/forums/showthread.php?t=229
Maybe your ISP is interfering on port 25 (SMTP).