so i was getting the same error on my new ispconfig3 install.
here's what i did. in my case, the problem only arose when i was making an email with the same domain as the server itself. ie. email@example.com
and my servers hostname is xxx.com. all other new emails could have email sent to them without the email being refused.
i simply removed my server hostname from mydestination in postfix's main.cf. i guess the postfix installation or the ispconfig installation set this. i don't know if this creates any problems otherwise, but everything works now. i had previously did a manual postfix setup on another server (without ispconfig), and the 'mydestination' field also did not have the hostname.
maybe this helps somebody.