No I could send mails using SquirrelMail and the mailboxes I created in ISPConfig to gmail but not to another e-mail adress. Then I got a message with the diagnostic code. I solved this problem by putting: smtp_never_send_ehlo=yes in main.cf
If I understand you correctly I am unable to use for instance email@example.com
or any other e-mailadress for my site if my ISP does block port 25.
Momentary when I use the same ISP from a different IP-adress, mail to firstname.lastname@example.org
is correctly redirected to the adresses I put in customrules. Using another ISP nothing happens. So this is due to blocking port 25?