Okay dudes, problem solved
I opened port 25 in my router.
After doing this mails sent to firstname.lastname@example.org
got boounced back. I found the following error in my mail.log:
Dec 4 18:32:00 blabla postfix/trivial-rewrite: warning: do not list domain myacc.webhop.net in BOTH mydestination and virtual_mailbox_domains
To solve this problem I did the following:
postconf -e 'mydestination = localhost, localhost.localdomain'
Now I am able to send and retreive mail on my own mailserver... whooohooo!