On my ispconfig server I had this occur. Amavisd was down that is why postfix wouldn’t deliver email. When I tried to start amavisd:
It gave me an error that my hostname wasn’t valid.
I edited this file : /etc/amavisd/amavisd.conf
Uncommented the following line and inserted in a proper hostname:
$myhostname = ‘something.yourdomain.com’
Save the try to start:
Send a test email , postfix shoud not delivery mail just fine.