Update:
I re-installed postfix, connect to localhost on port 25 via telnet. And I sent a test e-mail like that:
MAIL FROM:<noreply@domain.com>
250 2.1.0 Ok
RCPT TO:<my.name@gmail.com>
250 2.1.5 Ok
DATA
354 End data with <CR><LF>.<CR><LF>
Subject: Test Message
This is a test.
.
That Works and I did receive the email (in spam). But as soon as Im trying to connect the smtp server to my xenforo and try sending mails, I will not get any mails (not even in spam). There are also NO errors. Not even in /var/log/mail.log.
Could someone please help me. I dont get it.