550: Recipient address rejected: User unknown in virtual mailbox table

Hello,
I've configured a VPS with ispconfig and it works perfectly.
Among the others, I need to use ispconfig as a smtp server for one domain having an external pop3 server.
The domain is registered on an external dns and the mx records are configured correctly.
I'm also able to create a mail account like [email protected] on ispconfig and use it to send email.
The problem comes when I try to send an email to [email protected]. If I try this, I get the following error:
550, 5.1.1 : Recipient address rejected: User unknown in virtual mailbox table
I know that this may be solved creating an account like [email protected] on the ispconfig server, but I need to keep the username for the smtp unchanged, otherwise I'll have to reconfigure a lot of already installed software and user accounts on their pc's.
Is it possible to force ispconfig to look for the mx records instead of using its internal table, and only for that domain?
Thank you
Kind Regards.