Hide local IP from non delivery notifications

Hi everyone,

I have an external MTA configured with Postfix that delivers email to an internal IMAP/LMTP Dovecot server configured to bind an LDAP to check if users exist.

If the user does not exist, my MX sends a non delivery notification, and into the e-mail there's the local IP address of the Dovecot server.

I can't find a way to remove or hide that IP, here is an example of line that contains the IP (with X) that I would like to hide/remove:

*"The mail systemuser@domain: host hostname.domain.com http://hostname.domain.com[XXX.XXX.XXX.XXX] said: 550"*

Thanks in advance!

Cheers

-- Claudio